Technical frame coating - powder coat or paint?

When we built the Zipper, a local was getting ready to open a PC operation in Durango. The frame, running gear and wire wheels were sent to the PC shop. Reps from DuPont came down from Denver to teach the locals how to do it and for $340 worth of red and black powder, my car was powder coated. No chips, rust or lifting in 21 years. I’ve changed some suspension components and have had to PC some new stuff. Since I used black satin and cardinal red, everything has matched.

 

I smoothed out my frame with bondo. Hard to pull off with powder coat.

 

It sounds like several of you have experienced the same thing that I have -mechanically, when you powder coat the electrostatic charge creates a field similar to that of a magnet - when you look at electromagnetic field created by a magnet, there is none on sharp edges (look at a magnet that's been rolled in steel shavings). That is precisely why no powder goes on the sharp edges. My powder coating buddy says that he can bake the frame first to get hot in that will promote adhesion on the sharp edges, but not guarantee it. I am going to research this "powder etching before powder coating" concept.
I hear a lot of you commenting on how you've had frames powder coated 10 or 20 years ago and they still look great- that gives me some level of confidence in doing powder. I just keep referring to my experience with powder coating automotive parts - and that was all in California so it's not humid -and every part I've ever had powder coated (and it was thoroughly sandblasted and cleaned with acetone before coating), eventually peeled off starting at the edges.

 

Well, the first thing is you don't 'clean' with acetone as that leaves a residue that inhibits bonding. All I've ever done is blast it then blow any remaining dust off before coating. You can't touch any surfaces with bare hands after blasting or use anything made from petroleum. BARE and DRY is the key. A uniform fully blasted surface is also key; skipping 'clean' areas or leaving mill scale can lead to coating failure. Powder really needs that 'tooth' that blasting gives to adhere well.

If you're not getting edge cover, the coater is skimping on powder unless the edge is really sharp. It's nearly impossible to apply too much but easy to not apply enough. Given how cheap powder is it's a mystery to me why coaters risk a poor job to save powder. A full-size frame won't take more than about three pounds of powder to put a good coating on it and if you're using a 'standard' color won't cost more than about $30 for powder. I will admit that I'll 'break' any really sharp edges with a sanding disc before blasting to insure coverage. Sheared edges should get this attention.

Pre-heating the part is useful for insuring that powder goes into nooks and crannies or for getting a thicker film, but the coater needs to be careful as you can apply too much and actually get runs. This is also a good way to re-coat a part; once coated, the coating insulates the part so powder can't find the 'charge', the heat allows the powder to partially melt on contact and stick. Want a corrosion-proof battery tray? Put three to four heavy coats of powder on it and it will never rust...

Every coating failure I've ever seen was either an application problem (not enough) or a prep problem (much more common). I've got three keypad stations for a security gate that were coated about 25 years ago and after all that time still don't have a spec of rust on them despite of being in the weather 27/7/365. The coating has dulled a bit in that time, that's all.

There's two main differences between paint and powder. One, as paint gets thicker, it becomes more prone to chipping. Powder is the opposite. The thicker the coating, the more durable it gets. Two, paint can repaired fairly easily and can be buffed to a fine finish. Powder, once it's cured, is what it is. Repair is difficult and forget buffing it. Damaged coating usually means a recoat at the minimum. It does show 'micro-scratches' with handling/cleaning also. Remember those cheap plastic toys you had as a kid? Nearly unbreakable, they started out shiny but over time started showing scratches from play? That's what powder does... While powdercoat can look very good when new, remember that what you're really buying is long-term durability.

A few tips. One, I don't get carried away masking. Bolt holes or threaded holes I usually don't mask. Powder won't go that far into the hole, a quick pass with a drill bit (the exact size of the bolt shank) or tap will clean it out easily. Exceptions would be if getting to the hole would be difficult or it's an oddball thread. I found I ended up with a better job this way. I will mask any threaded studs, and any precision holes or mating surfaces; things like holes for bearings or where the coating thickness will interfere with proper assembly. Two, powdercoat is a pretty good insulator if applied right so if you're using your frame for chassis ground, weld in some threaded rod couplings in strategic places for grounding points. Clean the threads and knock the coating off the end of the couplings after coating and you've got your ground points.

 

One more tip. Want to make your coater smile and save money? Bring in the part with NO masking (by him) required. Over 90% of coating cost is labor and energy (for the cure oven), anything you do to reduce his labor charges is money in your pocket.

Tape isn't going to cut it as it won't survive the sandblasting, you need metal masks. Threaded hole that needs masking? Install a sacrificial bolt in the hole, discard after coating (although I don't recommend this unless absolutely necessary as it leaves a 'lip' around the hole). Threaded studs? Cheap nuts on the stud to cover the threads, same deal. Washers in bearing holes, etc. I've had multiple motorcycle frames coated and found that bringing a frame in with a 'just blast and coat, no masking needed' instruction cut the cost in half. I doubt you'd save that much on a car frame, but every bit helps...

 

This bridge was marginally prepared using wire wheels and grinders. Due to a lead paint hazard, it was overcoated with two coats of permanent rust sealer and one coat of 111 topcoat. An inspection 14 years later revealed less than 5% corrosion. The bridge is located in Pittsburgh, PA. Bridges done with this three coat system have generally a 25 year service life before repainting.
Sandblasting is recommended as it is your preferred Surface Prep. As far as edges go rust will not go beyond the edge. If you should manage to chip it , feather it out with 220 ,come back over
Even with a brush
Its as good as new. Less than a hundred bucks to do a chassis.

 

Can't say enough good about powder as long as it's blasted, plenty of product applied and not over-baked. I have a good relationship with my coater. My first coated item was a lumber rack for my work truck about 1980. I used it every day and couldn't believe how well it held up for the 20 years I had it. Worn down on the crossbars from sliding lumber on & off but the rest still looked great.

Within the last 9 years I'v had 4 car frames done along with tons of inner panels, wheels, brackets and anything else that can get it. The turnaround time is so much better than paint shop "jail". Long live powder coating. The better you protect the assembly points like threaded holes the faster final assembly will be. It's well worth the time investment and you get an assortment of colored bolts.

My current project, a 1934 Packard V12 limo, had the entire chassis including everything in blue coated while assembled minus the brake parts and drums as per owner's decision. He's up in years and wants time to drive the finished car. Actually turned out better than I expected and saved lots of money but was in very good shape to start with. Installed new bearings, seals, brakes, etc. after coating.

After a power wash



The day it came back.





Couple weeks later on the way to paint shop for body.

 

Thanks for the tips - I've followed all of those in the past and have still had bad results (will post photos later) - coater said the only way to guarantee no corrosion underneath is to zinc plate the parts first. I'm still contemplating powder or paint - but since I don't intend to drive this much and never in the winter/salt, I'm leaning toward powder because it'll be easy and done quickly.

Earplugs work great to mask threaded holes!

 

man, that's kewl !

 

Find another coater then... He's not blasting the part clean and/or applying enough powder.

 

I always paint, but never with a spray gun, or rattle can. I use small rollers and brushes to paint the frame, and floors inside and out. It goes on heavier, and lasts for longer than I can say. Have one I did 11 years ago, with many thousands of miles on it, and I pressure wash the undercarriage every spring, and it's still perfect.
I use Rustoleum gloss black, or mix 50-50 gloss and flat to get satin if I want a satin finish. I do use a little Rustoleum rattle can paint to get to tight areas where I can't get a paint brush into, but that's very few places.

 

So here's something to keep in mind when thinking about powdercoating a frame. On the '31 only 2 things are painted... the engine block and frame. Everything else is PC including trans case, 9" housing, drop axle, etc. Paint and PC are both black metallic, but there's a different look between the 2. I'm not so concerned as I'm looking for the durability of PC for suspension components but the flexibility to make changes to the frame (if need be).

 

When I powdercoated my frame you couldnt even buy primer and paint for the price my parts were done for. Was a simple answer of economics then. And the durability is great too. yeah, youd better have it all the way its going to be, as its not so easy to touch up. but thats fine, my car is a show car. one and done

 

I love powder coating. On a driver I PC all the chassis and running gear, engine, fender wells, tranny case and even exhaust past the headers. It can stand that heat. One aspect that gets overlooked, PC parts get a clean sand blast which you should be doing anyway so when factored in makes the PC cost really attractive. Drawback is if you need to weld after coating, will violate the seal. I have frames that are 20 yrs old and zero chipping, zero rust, still wash up and look like new.

 

I like powder coating because it looks very nice and is durable and more scratch resistant. The downside is powder coat is usually pretty thick and flexible so it can hide cracks in highly stressed parts such as frames and suspension parts. If moisture gets under the powder coat because of its thickness there is no bubbling on the surface until a large section peels off or the part fails due to corrosion.