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Technical Sand blast, bondo & firewall ?'s

Discussion in 'The Hokey Ass Message Board' started by lostone, Sep 6, 2021.

  1. lostone
    Joined: Oct 13, 2013
    Posts: 1,794

    lostone
    Member
    from kansas

    Ok got the firewall on my 31 done and the holes all welded up now I'm ready to Sand blast and body putty and prime.

    The primer states bondo under primer straight to metal and I'm happy with that so my questions are....

    1- I'm using black diamond medium grit media from tractor supply to blast with and will this give it enough tooth for the bondo to bite too?

    2- plan is bondo, dtm primer, filler primer on top of dtm primer then base/clear paint. Sound good enough? Paint supplier said it should hold up just fine but its been 15 yrs since I've done alot of painting and alot has changed!!

    Here is where I'm at. The trans tunnel is screwed down on edges and slides up under lip on front. So you just remove screws and slide tunnel back 1/2 and it comes right off. Making the tunnel and lip was a major pain but came out great...
     

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  2. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,419

    oldiron 440
    Member

    I would sand the blasted surface with 80 grit then hit filled areas with 24 a grit grinder, my reason is i want a smooth area to feather filler to. That's my preference after 45 years.
     
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  3. squirrel
    Joined: Sep 23, 2004
    Posts: 49,836

    squirrel
    Member

    I might run a DA with 80 grit over it after blasting....
     
  4. Most new fillers do not like coarse grits. Hit is with 80 grit and you will be good to go.
     
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  5. krylon32
    Joined: Jan 29, 2006
    Posts: 7,509

    krylon32
    Alliance Vendor
    from Nebraska

    It's been my experience the last time many years ago I did a firewall like your doing if the grinder marks are to deep they can come to the surface to haunt you?
     
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  6. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,966

    Bandit Billy
    ALLIANCE MEMBER

    I am at the same stage on mine as well. Mine was blasted and primered prior to me buying the truck but several years ago, so I just had to sand it all off, do my welding up of about 80 holes, sanded with 80 with DAs, filler is Evercoat Ultra, going over it with Omni epoxy primer. I'll scuff the surface of the Omni with red scotchbrite to break it and give the Evercoat Ultra high build primer something to stick to, block then base/clear. I am lucky we have had a very nice, dry summer up here to keep the flash rust out of the equation.
    upload_2021-9-6_11-16-22.png
     
  7. lostone
    Joined: Oct 13, 2013
    Posts: 1,794

    lostone
    Member
    from kansas

    That's one thing here in the corner of kansas I've been fighting is high humidity and rain.... Lots of both and surface rust is a b*tch!

    My plan is to blast and filler one day and prime the next.

    Yep lots of holes welded up too !!

    Left the center hole so I can build a nice little cover to bolt on there and run the wiring thru it, under the tunnel ram and forward to the distributor and alternator. Keep the view of wiring to a minimum.

    Its nice to get to this point but I'd like to be alot further along. Haven't touched it in 3 weeks due to back pain but it ain't getting done sitting there ! LOL !!
     
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  8. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,966

    Bandit Billy
    ALLIANCE MEMBER

    Stay at it, rust doesn’t sleep. I don’t know if it is necessary or not but after I send blast a piece I take my propane torch to it just enough so you can see the water dry off the panel.

    I used to have more water in my air system than I do now, so maybe that was the cause.
     
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  9. lostone
    Joined: Oct 13, 2013
    Posts: 1,794

    lostone
    Member
    from kansas

    Thanks bandit billy ! That's a great idea! I will be sure to follow thru with that tip.

    Yeah my compressor is great at generating water right now! Its becoming a down right problem !

    My water separator is having problems keeping up so I guess thats the next tool to build for the shop! A BIG water separator and drier.... Hate it too, because thats time money and energy I could be putting into my car!
     
  10. Squablow
    Joined: Apr 26, 2005
    Posts: 15,368

    Squablow
    Member

    I personally would pound on that firewall with 80 grit DA paper as others have said before doing any filler work, also I find that sanded sheetmetal won't flash rust as fast as stuff blasted with black diamond. I think the blasting stands up little tiny "hairs" of the steel that rust easily. Filler seems to smooth out easier for me on sanded surfaces, too.

    Blast, sand, fill, block, then prime would be my suggestion.
     
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  11. SPEC
    Joined: Feb 1, 2021
    Posts: 360

    SPEC
    Member

    Sand blast it then Bead blast it to smooth it out.
     
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  12. That's how I have done cars & firewalls. HRP

    [​IMG]
     
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  13. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,966

    Bandit Billy
    ALLIANCE MEMBER

    It isn't a cheap solution but I purchased a Ingersoll Rand electronic air cooler, basically a heat pump for my air system and I have zero water and no more desiccant beads or filters to worry about. I still use a separator and desiccant chamber when I paint but I haven't seen water in this system since I installed it. About $1200 though but cheaper than a repaint.
     
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  14. Dave G in Gansevoort
    Joined: Mar 28, 2019
    Posts: 805

    Dave G in Gansevoort
    Member
    from Upstate NY

    Just a thought. For a one time project, rent a cylinder of nitrogen from your welding supplier. Inert and dry. Pretty cheap for a one time project...
     
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  15. anthony myrick
    Joined: Sep 4, 2009
    Posts: 8,688

    anthony myrick
    Member

    I always sand with 80 after blasting for body panels.
    I don’t like the “metal residue” left from blasting.
    Newer formulated body fillers like 80 grit.
    The cheaper, thicker old school versions like a courser grit normally.
     
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  16. oldolds
    Joined: Oct 18, 2010
    Posts: 3,189

    oldolds
    Member

    Hang about 20 feet of 1" metal pipe on your wall, bends are ok. Air compressor at one end, water separator at the other end. That should cool the air enough so your water separator should work. Hot air holds water.

     
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  17. squirrel
    Joined: Sep 23, 2004
    Posts: 49,836

    squirrel
    Member

    I had about 80 feet of pipe running around the top of the walls inside my old shop, and a few drops with drains below the quick disconnects. Never had any water issues.
     
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  18. I did a Harbor Freight dryer 5 years ago and zero moisture in the lines since. Tons of super humid days up here in the summer. All in was just over $400.00 to my door with the 20% off coupon and it always helps to have a best friend High School buddy who's a master plumber. 100_2188.JPG
     
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  19. HSF
    Joined: Oct 25, 2014
    Posts: 155

    HSF
    Member
    from Lodi CA

    Like almost everyone has said, hit it with 80 DA then do your filler work. Only thing I can suggest is don't use two different primers. Put 2-3 good coats of a dtm surfacer like house of kolor kd series after you are happy with your filler. Let it dry a day or two then block it with 220 followed by 320, wet or dry. If you aren't using a sealer, finish with 400-600 wet and spray away.
     
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  20. lostone
    Joined: Oct 13, 2013
    Posts: 1,794

    lostone
    Member
    from kansas

    Actually I bought some nason, buddy said he has had good luck with it. Bought their DTM primer which is an epoxy and their filler primer made to go on top which is a urethane. Supposed to be high build and easy sanding but we all know that if we had a dime every time something was supposed to be easy or a perfect fit we could all afford to have our cars custom built for us! LOL
     
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  21. squirrel
    Joined: Sep 23, 2004
    Posts: 49,836

    squirrel
    Member

    I used Nason urethane primer on the last few I've done, it works fine. I don't bother with epoxy primer....but then I don't build high dollar cars that have to survive sitting outside in the tough northwest winters, or along the coast.
     
  22. KenC
    Joined: Sep 14, 2006
    Posts: 646

    KenC
    Member

    Contrary to all the advice you've gotten so far, my process is just wash the blasted metal with wax and grease remover. Then apply epoxy, do the filler work on fresh epoxy then filler primer. Saves the sanding labor and works for me. The layer of epoxy also helps show where filler is needed better than bare metal.

    Epoxy first seems to be what most of the manufacturers are recommending now. But either works since filler on metal has been used since Bondo was first made.
     
  23. He said in his first post that filler cannot go on top of the primer he is using.
     
  24. 54chevkiwi
    Joined: Jun 28, 2020
    Posts: 276

    54chevkiwi

    theres literally no need to wax and grease remover the freshly blasted steel, its perfectly clean blasted steel ready to epoxy. And a pain in the ass to wipe off with lint getting all through your blasted surface

    i agree 100% with you though on the epoxying it first.
     
  25. 54chevkiwi
    Joined: Jun 28, 2020
    Posts: 276

    54chevkiwi

    find an epoxy like SPI to put straight over the metal before bondo..
    Realistically, with life, you wont get the bodyfilling done on a firewall in a day.. it just wont happen
    So to blast, quickly rush the bodyfilling in a day and THEN prime before humidity rust sets in probably wont work..

    blast, epoxy, THEN you can muck around as long as you like getting it bodyfilled and it wont rust.
    Sanding the blasted firewall first then sanding the primer again to carry on is double the time you could spend elsewhere, so just hit the blasted steel straight with the epoxy.

    not sure the type of primer youre using but generally the recommendation to body fill the steel is old tech with old products and a manufacturers CYA...

    modern stuff prefers bodyfiller over the top..
    The reason they say to put bodyfiller over the steel is because with old single pack paints like enamel and lacquer when they were used, if you dont grind it off and put the filler over top, you end up sanding the paint to steel around the repair while blocking it and then the solvents in the next layers fry up the stuff under the filler and you get that awful halo in your paint job, getting it to steel first prevented that...

    so, with the modern stuff available now, you go straight over the epoxy, SPI is great, he gaurantees it used under and over anything.. not like other paint companies that fear monger you into only using their system. Its a polyurethane and doesnt need MORE money spent on something to go over top.
     
  26. Tman
    Joined: Mar 2, 2001
    Posts: 34,941

    Tman
    ALLIANCE MEMBER

    There is a great thread or two over on thegaragejournal.com showing how to make your own cooler
     
  27. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 2,108

    ekimneirbo
    Member
    from Brooks Ky

    When air is compressed it becomes hot and holds moisture. Pump your tank up and put your hand on the tank to see if its hot.
    Most people place their water separator right on the compressor where it is virtually useless. Air needs to cool before going thru a separator. I have heard 10', but my experience is that the farther the better.
    On my system I bought a used 80 gallon tank but a 60 would probably be ok. I used it as a water separator.
    My initial compressor/tank pumps the air but instead of going into its tank, I routed it to a secondary tank. Air enters the secondary tank thru the original hole in its side. Thats about midway down. The air expands and moisture can drop to the bottom. The somewhat cooler and less moist air rises and exits the top of the tank and then returns thru a line to the tank under the compressor pump. A used tank can be had for $50 or so.

    After the air has returned to the compressor tank, then it is routed thru a wall to the paint booth where I bought an industrial pressure reg/water separator. for about $125 on Ebay. The lines running from the tank to the water separator and on to the spray gun connector are larger lines to allow lots of air flow at low pressure. Thats why I bought the industrial water separator, because it had larger pipe fittings.
    Thats the simple explanation........and its pretty cheap to do. The secondary tank can be outside if space is a problem.
    It takes a little work to add an extra tank, but it works well and isn't overly expensive.
     
    Last edited: Sep 7, 2021

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