View Full Version : Block sanding tips

rat bastad
05-17-2004, 05:00 AM
Hey guys....

I'm in the process of block sanding my panels on the 3W. All the hanging panels and body are in two pack epoxy primer and have been guide coated. I flow coated the primer finish which is very flat with no orange peel to facilitate block sanding.

What I really wanna hear from u bodywork guys is....what methods do use to get the panels (esp the doors) as straight as possible.

Im aware of the long/hard block method but more info would be great before I get started. I'll be using 400 wet & dry to block back the guide coat and then 600 to prep for top coats....unles you guys tell me otherwise

What say the HAMB body guys ? Overspray ?

05-17-2004, 06:55 AM
The guy that taught me to block-sand cars showed me how well a straight 14-inch oak one-by-two worked. That's the way to get those panels free of ripples and waves. I use different lengths of blocks, ranging from eight inches to two feet, for different areas. I use two-inch-wide, adhesive-backed sandpaper, wrapped around three sides of the block.

After you have your black guide coat fogged on, you have to sand in three different directions. First go at 45 degrees from the bottom right to the top left; then from the bottom left to the top right, and then from top to bottom (or front to back). In each stage you have to sand the entire area, and you must never put more than about a pound of pressure onto what you're blocking out. The way to get great results is by not compromising and not rushing or cheating.

Four years ago I wrote an article on block-sanding but didn't add it to my site because I haven't taken any pictures for it. I'll dig it out and finish it soon, but what I said above has the most important guidelines.


05-17-2004, 08:35 AM
There really is so much to know about doing this job right, that I don't know if I can express it all in a short post!
You are on the right track to begin with, Dave's suggestions are correct, also. Always use the longest block you can on the panel you're working on. I always start out with straight front to back lines, then switch to the 45* line, and then go 90* to that. Work in an X, like Dave explained. Finish with straight front to rear. That is the plane you will see the waves most in, so spend more time sanding in that direction. Use long stokes, even if it's from one edge of the panel to the other.
It is also important to hold the block correctly. A lot of guys, when doing the 45* cuts, hold the block in the line that they are sanding. That is to say, pointing at a 45* angle. It works better if you hold the block in the front to rear orientation, even while doing a 45* cut. When doing the side of the car, keep the block parallel to the ground, even when sanding from left bottom to right top, or the opposite way, left top to right bottom. by keeping the block "straight" your body will be straighter. Pointing the block in the direction of the 45* cut 'might' just put flat spots in at a 45* angle, instead of making the panel smooth.
Use other things to "block" out different shapes on the car. A flat block is still OK for a convex panel, or sure to use the X pattern though so you don't sand 'flats' in it. I also have a variety of teardop sanders, radiator hose, heater, and fuel hose, paint sticks, etc., to get into curves, edges, and sharp corners. be creative. I use the backside of the sanding block to do mild concave surfaces, like where sail panels meet quarters on old cars.
If the car has had a lot of bodywork done to it (as all my Kustoms have!) you should do 2 or 3 blockings. I start with a dry 100 grit block sometimes, or with a 180 wet. Primer again, guide coat, and do a 220 or 320 cut. Then reshoot with slightly reduced primer, guide coat, and a 400 or 500 cut, depending on the type of paint going on top. Enamels, SS urethanes, can even go over a 320 cut. Base clears need 4 or 500, while some pearls or transparent metallics need a 600. Different brands of paint seem to require different cuts, as well. My Dupont Chromabase doesn't mind a 400 finish, while PPG basecoat looks scratchy even with a 600 finish, but smooths out with the clearcoat.
You don't want to start out with too fine a grit to begin blocking, 400 is pretty fine! A fine paper will only smooth over the waves, without cutting them straight. That is why I use a rougher paper to beging with, it will cut the panel flat, not just smooth the primer. There is a difference!
To try out your technique, you can even spray the panel black to see if you've done a good job. I do this sometimes on candy finishes, to see if the car is straight enough for the expensive paint. Black is cheap. It WILL show any waves, or imperfections. I'm not saying to do the entire car, per se, just do a panel that had a lot of bodywork on it, for example. That way you'll see if your sanding technique is working to get the panel straight, if it is, you can be pretty sure the whole car will be OK.
Good luck!.....and as my buddy Rusty used to say, while doing wetsand blocking.."if you ain't wet and miserable, you ain't doing it right!"

05-17-2004, 08:45 AM
From the picture, it looks like you are a way better bodyman than me--nice looking door!! Dave is exactly right! I'm usually in a hurry and rely on the 2 part fill primer more than I should. My work needs coarser grit than 400 to find the ripples. It always seems to take me longer, than other guys I know, to get panels straight (thank goodness for the popularity of suede paint). 400 doesn't cut enough for me. If you have what's under the primer straight, it will go smoothe fast. I use 180-240 grit (dry) for my first primer block, then after I reprime spots or whole panels, I switch to 320-400 grit (wet or dry). overspray

05-17-2004, 09:03 AM
Ya-what chopolds said. That is "gospel" from a very talented and reputable craftsman. (That's the way I read his post). overspray
05-17-2004, 01:47 PM
I'd like to stress the importance of never putting more a pound of pressure onto the sandpaper.

The whole point of blocking is to bring down the high spots and get them to be at the same height as the low spots, resulting in something with uniform, gradual contours. The larger the radius of an imperfection, the more-easily the eye is fooled into perceiving it as being flawless.

Any distortion created by using too much pressure completely defeats the purpose of block-sanding. It's often very tempting to sand out a low spot by varying your technique, and that is a bad mistake. When you locate a low spot, you work around it, and feather out the area surrounding it. Better yet, massage the panel, or add some more primer, and then sand the entire area again.

It saves time if you keep a roll of 3/4" 3M masking tape with you as you sand. As you're blocking, when you notice a low spot that's going to need to be brought back up with more primer, just put a piece of tape in its center and keep blocking. When you've sanded the whole thing, then you can go back, remove a piece of tape, scuff it with coarse Scotch-Brite, prime it, and continue until all of the low spots have been built up.

It's good to always hold the block with three fingers at each end. Whenever possible, I hold it parallel to the ground. If you're going front to back, keep it at ninety degrees to the ground.

Before you block doors, hoods, fenders, and trunk lids, put them on the car, with all of the hinges, fasteners, stops, and weatherstripping in place. Otherwise, you won't be able to get everything to flow near the seams.

Wear a dust mask.

I use a clean rag, or a new cotton baby diaper (which is lint-free), to keep the work clean, and then sweep up. Using the air nozzle just puts all the dust somewhere else in the shop.

I'd generally start with 240-grit paper. Every time you block out a car you will notice low spots you didn't know were there.

Having a second pair of eyes to check the work is a good plan.


05-17-2004, 02:06 PM
To try out your technique, you can even spray the panel black to see if you've done a good job. I do this sometimes on candy finishes, to see if the car is straight enough for the expensive paint. Black is cheap. It WILL show any waves, or imperfections.

[/ QUOTE ]

That's a great idea - never thought/heard of that!

05-17-2004, 02:06 PM
<font color="green">Damn!

This is some good info.

I never thought about the amount of pressure applied to the sandpaper.

I was putting my all into it.

Shit, with this info, I may be able to get a car ready for SHINY paint.

I'm copying all this down and saving it for when I have time to do some body work.



Now, back to some O/T posts!!! </font>

earl schieb
05-17-2004, 02:09 PM
Before you block doors, hoods, fenders, and trunk lids, put them on the car, with all of the hinges, fasteners, stops, and weatherstripping in place. Otherwise, you won't be able to get everything to flow near the seams.

[/ QUOTE ]

That's one thing FOR SURE that needs to be done! If the adjacent panels aren't in the same plane, the reflections in the final finish will "skip" from panel to panel. A crisp reflection that carries from one end to the other sets you apart from the masses every time.
I start on most cars(assembled) with 80, prime/block again with 180, repeat with 240, then disassemble for 400-600 blocking with a semi-soft pad. Hell, I even block clearcoats most of the time

Just one that epoxy primer or primer-surfacer? Real epoxy primer won't get you where you need to be.

Boy, are you fixin to have some FUN!

05-17-2004, 02:09 PM
so let's say you're sanding a horizontal surface, like a roof or a hood. do you sand from door to door or front to back? i'm guessing the later, but i thought i'd ask.


earl schieb
05-17-2004, 02:15 PM
ed, on a compound curve, I (cross)sand in the direction that places the largest area of sandpaper against the panel. Works for me, so it's probably wrong

05-17-2004, 03:34 PM

so let's say you're sanding a horizontal surface, like a roof or a hood. do you sand from door to door or front to back? i'm guessing the later, but i thought i'd ask.

[/ QUOTE ]

It should be from one corner to the opposing, like chopolds was sayin. Then the opposite, then either front to back or side to side...or both! 3 directions at least, 4 is ok too. It's important the block is NOT pointed in the direction you're sanding, it will cut in at the sides and carve lines into the surface. One thing also is at this stage, light pressure is very important but equally important is to use FRESH PAPER. Use alot of it. When it's half worn CHUCK it and grab a fresh peice. Worn paper will waste your time, and you'll be working backwards pretty quick as the worn paper will simply be smoothing out the waves insteada cuttin em, as stated above. So, multi-directional sanding patterns, lighter pressure and fresh paper. Check the panels against the reflection of the lights (assuming they are florecent booth type), and check by feel WITHOUT looking. Sometimes a dip looks like a hump or vice versa, but touch only without looking will tell the truth...

some of my hackin' >>>

05-17-2004, 07:10 PM
I'm in no way a pro at blocking or painting, but I've prepped and painted most of my hotrods and nobody has ever dogged on my work. I have found that blocking with anything finer than 220 is a waste of time. Most of the time I finnish up with 320. If you don't use the right cut paper you don't cut, you blow over and don't get what you want. You really need the bite that 320 gives you for most paints. I agree that there is sometimes with certain paints you should go to 400, but beyound that I have never needed to go except for color sanding and rubbing.--TV

05-17-2004, 09:03 PM
Tech-O-matic please.


05-17-2004, 10:16 PM
Would Black primer work to show ripples and waves in the body panel when it is wet?

One more vote for the Tech-O-Matic.

05-17-2004, 10:20 PM
I use a stainless ruler the length of a piece of green stickit board paper..fold it lengthwise and it covers both sides of the ruler. Use cheap cotton buck for two pairs at walmart...and it gives you almost a velco grip on it. Start with eighty grit and keep on the bondo till its good...fill with hi build primer and continue with eighty grit till its nice and flat....the steel rule does a great job on low crown areas. The term "flat"...doesn't really mean means smooooooth with no imperfections and the steel rule will give you that.

05-17-2004, 10:30 PM
All I know, is that I am at this stage. I know what good work is, I know how to get there. But I want to drive this car soon. My timeframe might make me put mine in sealer

Great post, good tips.

rat bastad
05-17-2004, 10:41 PM
Wow guys....fantastic info...great responses.

Yeah...I was planning on block sanding the same way.... x method plus up and down and using light pressure letting the papper do the cutting.

Im also aware of how important fresh paper really is too. On a 3W coupe....the only real flat/slightly curved panels are the doors, trunk lid and roof. All the other panels are curved and are of a much smaller area.

Would a smaller hard block using the x method here be OK whilst using even smaller blocks to sand into the body lines properly? Thats what I was planning on doing here.

Does it make a big difference using wet sanding instead of dry? I prefer wet to keep dust right down.

Any comments ? Thanx for the info....its killer....keep it comin' !! Ill post pix on progress....


05-18-2004, 08:27 AM
Great info guys I'm in the process of block sanding now, and I now now a lot more than I did a few minutes ago.

05-18-2004, 09:51 AM
Great info guys tech-o-matic for sure, sorry if its already been mentioned but i always wet sand with soapy water helps stop the paper clogging up, also worth investing in some tac rags to clean the panel before paint.

rat bastad
05-19-2004, 12:56 AM
Well here's some progress following the fantastic tips I got on here. Will post pix later....

I guide coated the door, blocked with 220 in the correct manner, found a couple of small low spots which were sanded very lightly for primer adhesion, then reprimed with hi build primer and guide coated again, blocked again with 220 then 320 then 400 to take out the scratches.

Came up real sano. One thing I found.... I had to resist the temptation to block out the low spots and went with the repriming/filling/blocking method instead. Takes heaps more time but I think the end result is worth it.

One trick I learned was to wipe the panel down wet with wax &amp; grease remover leaving it "wet" then by sighting up &amp; down the panel in flouro light, any waves will stand out. Worked for me anyway !!

Have one more question though. How do you find sandpaper long enough for a 14" block? I figure you can use two pieces of paper....but won't leave a line in the primer ?

What gives ?


05-19-2004, 01:09 AM
Most good paint shops will carry the sandpaper in rolls. They come in a variety of grits, and are sticky backed. It's what, 2 inches wide? Same width as most blocks...and it will fold in half over a paint stick, which I use on some areas. Make sure it is a good quality paint stick, or stack two together. Works for me.
Tech-o-matic vote for me also. The cotton gloves idea is worth a thousand blistered fingertips.

For wet sanding, I use an old car wash sponge. I dip it, and hold it above the work area and let the water slowly bleed out, or give it a gentle squish from time to time. A dish detergent bottle with the pop up cap can work good also.

Guide coating the bondo before te primer will help a lot. I like to dry block the first round of primer, and wet sand the second when I'm real close.
I also wipe down the area with wax and grease remover and check for waves against a light.

Slag Kustom
05-19-2004, 01:58 AM
i always finish my plastic work with 400 dry paper guide coating between paper changes to get the best results and not having to worry about having and sink back or swelling from using too much primer to fill the scratches. the longer you can let the primer dry before you sand and paint it the better your finish will be. also be careful with the way you guide coat if you put heavy lines of guide coat some times you can see the efect in the paint from the solvents in the guide coat (i wrote fuck you in guide coat on a costomers car and in certian light you could make it out)

05-19-2004, 04:15 AM
also be careful with the way you guide coat if you put heavy lines of guide coat some times you can see the efect in the paint from the solvents in the guide coat (i wrote fuck you in guide coat on a costomers car and in certian light you could make it out)

[/ QUOTE ]


05-19-2004, 05:47 AM
hey rat... bunnings sells sandpaper right down to 40 grit by the roll and by the metre, and i imagine many other places would also. i've got a couple of 14 inch blocks (and a 14 inch airboard - yeeha!) and i get my paper there.

with your wax and grease remover, i hope you checked the labels because a lot of them say you shouldn't allow it to dry on the surface or it will leave shit behind. you have to wipe it off before it evaporates... worth checking mate. can't wait to see 'er shiny!

05-19-2004, 07:40 AM
OK, To summarise for the Tech-o-matic:
180 , 240, 360 paper. I recycle mine (240 soon becomes 360!)
hard rubber sanding block for general use (5x3" face).
hardwood block, 2x2x14" (the longer the better on flat panels),plane the wood dead flat with a slight 1/16 rad on the edges so it doesnt leave accidentale lines/cuts.
I use full length sheets of wet and dry cut to 3" wide folded up each side and held onto the block with fingers front and rear.
1 bucket with plenty of clean warm water (change frequently to taste) with a few drops of liquid soap added to break down the water tension, easy does it , yer not washing dishes.
1 cheap rattlecan of acrylic black paint (for guidecoating)
1 rattle can of etch primer
1 nice toweling rag.

Guidecoating is a must, dont attempt to sand a panel flat without guidecoat, and guide coat every time you lay on more primer/filler etc.Mist over the whole panel.
Always wet the panels and the block/paper before the first stroke of the sandblock, rinse panels and block/paper very frequently.
If you get down to steel in a spot do not sand further(its a high spot and will need some panel massage).
Continued sanding over steel show-throughs will give you flat spots which are worse to repair and will easily show under paint.
Use the towel to wipe up and clean panel during sanding.
keep everything fairly clean, it does help with panels such as hoods/roofs.
Always datum the length of sanding block following the longest flat surface. Keep sanding blocks horizontal in car position on doors if possible and move the block at a 45 angle to the floor.
Use light pressure / or heavier paper what ever yer fancy.
Never, Ever block sand using the palm or fingers only . You will get tram lines. Prior to further primer application always touch up bare steel spots with etch.
GO OVER THE COMPLETE PANEL at every stage of sanding paper, i.e. dont try to get a good result in small areas. the guide coat will disappear quite quickly after a few passes as you get the panel to the finished stage.
I could go on, but where do you stop?
For those of us that prefer shiney paint, WET block sanding of top coats is also a must.use the rubber block wrapped with cloth as a backing for 800 - 1000 paper prior to top coat.


05-19-2004, 07:49 AM
Don't think that these have been mentioned:
Check the surface by rubbing over it dry with a light cotton glove or a thin clean cloth. Your hand is usually either calloused from sanding or numb and the cloth seems to amplify the surface imperfections. Also, wet the surface
and with a long bondo paddle, scrape the water off. It will still cling and reflect on the low spots.

Erik B
05-26-2010, 02:58 PM
I did a search and found this to get some tips on final block sanding. Great info and since it's 6 years old I thought it should get a fresh bump. If anyone wants to add more tips please do.

rat bastad
05-26-2010, 09:26 PM
Nice to see my old thread has surfaced again hehe !!


05-26-2010, 09:46 PM
I got these.Expensive but helpful.


Erik B
05-26-2010, 10:29 PM
Yeah, those are rather pricey. I found a 18" flexible sanding block with a spring steel base from, you guessed it, Harbor Freight. Works pretty damn good and it was cheap. I'm going to cut up some polycarb 1/8" sheet for some variable flex sanders and see how that goes.

Thanks, Rat Bastad- a good one.

05-26-2010, 10:36 PM
Very cool post.

05-26-2010, 11:11 PM
I always finish my plastic with 80 grit, feather the paint,then prime and guide coat. Block with 150 grit free cut paper, re-prime and then finish sand with 320 grit. This is the way I've been doing it for almost forty years, everyone learns a different way.

Flathead Fever
05-27-2010, 12:29 AM
I took an auto body class at San Bernardino Community College thirty years ago that was taught by an instructor that had been doing body work since 1948. Lloyd Hammond was his name and he was the finest metal man I ever knew. He had a cross peen Proto body finishing hammer named “Ike“, the head was polished like a mirror so it would not leave scratches in the surface of the metal and it was on its third or fourth handle by then. When Lloyd was done metal finishing a panel all it needed was a light coat of lacquer primer, wet sand it with 600 grit and when it was painted it was straight like a mirror.
Lloyd showed us how metal finishing was done. I was the best in the class at it and I sucked! After several weeks of welding, brazing, hammering, stretching, shrinking I finally realize my limits and eventually gave into the concept of using body filler. You can only grind and file on something for so long.

I was taught by Lloyd if you were a sorry-ass-body-man (which I am) and you had to use body filler you should grind the metal so the grinder marks were lengthwise, in the direction you would normally being looking down the panel. Apply the schmooze (you could not call it Bondo in his class because that was a trade name) Sand the schmooze with 36 grit speed file paper in a crisscrossed pattern until you just see the metal starting to breakthrough around the edges of the dent or low spot, STOP sanding, do not try and make the schmooze smooth and pretty, all you will be doing at that point is creating a low spot. Just barely sand the schmooze with 80 grit paper to knock off the sharp edges from the 36 grit, your final sanding strokes should be lengthwise. If you have a hard line between the metal and the filler you have sanded too far and have created a low spot, you need another coat of schmooze. If the schmooze feathers out into the metal your probably still too low but your at the point where you can probably block it out with two, three or four... coats of primer.

I don't want you to think were sculpting a car out of schmooze, if it’s more than a 1/16” thick you need to go back with your hammer and dolly and work on it some more. The trick is learning to know if it needs another coat of schmooze or of it will block out with the primer. Run the flat palm of your hand over the body work, if you can feel anything at all it probably needs another coat of schmooze. The waves that you are going to be blocking out with primer are ones that are too small to feel with your hand but big enough to see when they are painted.

I agree with block sanding with fine paper, it will just ride up and down on the waves and the panel nevr seems to be getting any flatter, all your doing is wasting time and primer. I do all my blocking with 220 grit paper. Don’t let the paper get creases in it or build up on it. I sand with very light pressure until I hit metal then I give it another coat of primer and do it again. When I think I have it straight I give it another coat of primer and wet sand it with 400. Then I clean and dry the panel and take a paper towel and wipe it down with wax and grease remover. Before It evaporates it will look like the most beautiful glossy paint job you have ever seen. Look at it from all angles and if there is a wave in it you will see it. Then start cussing, go buy another gallon of primer and start the whole process all over again. It’s all about patience and how much primer you can afford.


05-27-2010, 12:51 AM
A couple more tips that work for me.

Block with a course paper, ( I use 100 grit), and as soon as you see the metal (or bog), starting to show STOP!! Any more blocking will just sand all the filler out from around the 'high" and make it worse. Allow enough primer still left there so you can smooth over with 180 grit.

And the big tip.....As you will know, that once you have sanded through your guide coat you are sanding blind, having to remember where the guide coat had showed up the high and low spots. So, simply tint differently each primer coat so that you have a "continual guide coat". It will also show if you are sanding a "flat" onto a curved surface.

I learnt this one trying to get the front of a set of 38 Ford guards (fenders) straight. It was like trying to block a ball!!

The Hank
05-27-2010, 07:19 PM
Here ya go buddy , I asked the same question a while back and someone posted this link.