I am going to re-do the paint on a tool box that the owner had painted to match his new truck, the problem is this thing is powder coated and the paint he had done is peeling off in sheets after only a couple of days. I thought powder coat has just to be scuffed and sprayed much like anything else and I have not had any problems in the past the few times I did paint it but the guy that did this says he will not fix it because he warned the owner it might not stick. The plan here is to remove the peeling paint then CLEAN THROUGHLY since I suspect some type of surface contamination then re-sand with 400 before re-shooting this thing. Is there any special prep required for powder coat? Any particular primer/sealer that works best? Any special steps for painting powder coat? I intend to use Nason single stage urethane.
Painting powder coat
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If it is a alum. box,I would try to remove all the old paint. At work if we would powder coat alum. we would scuff up the alum. to get the shinny coat off,to a dull coat,and make sure it is free from the factory oils,than coat with the powder coating,alum. will heat up faster in the oven,and will get to the flow stage faster than metal. They probley didn't scuff the metal to get the shinny coat off. hope this helps.
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It is possible to recoat over most powder coatings. The problems you observed earlier could be due either to contamination, or improper selection of the topcoat, or (less likely) a very unusual powder that is more difficult than usual to recoat.
Certain tool boxes may be more difficult than most articles to recoat successfully, as there have been known to be tool box manufacturers that require extreme chemical resistance when specifying their powder. (Others should be a piece of cake to recoat.) Having said that, it should be possible for you to get the job done succesfully, albeit with a little more effort than usual. Here's how I would proceed:
1. Remove the peeling topcoat completely and thoroughly.
2. Go ahead and sand the powder coating surface thoroughly. I'd use something fairly coarse, probably around 120 to 220. Finer is less advisable for what you're trying to do here.
3. Your good candidates for liquid topcoats would include either epoxy-polyamide primers, or a 2K urethane. If you should wind up sanding through the powder to bare metal in a spot or two by accident, then you might lean more in the direction of the epoxy. You should do a small test spot in an inconspicuous place to see how the adhesion is going to be on the particular product you wish to use before you paint the whole thing. This is a critical step- don't skip it! If you have multiple choices, your best chances of sucess will probably be paints with fairly strong solvents, preferably including some ketones like MIBK (methyl isobutyl ketone), MAK (methyl amyl ketone), or (MEK) methyl ethyl ketone, or esters like butyl acetate. I would definitely stay away from single component basecoats as your finish directly over the powder (some do work sucessfully, but your chances of sucess are improved with a two component product, all other things being equal).
4. Let your test spot cure thoroughly (best several days to a week at 70F or above), then take a knife and cut an X through the topcoat. Put some stong tape on it and check the adhesion. A proper choice of topcoat should be one that gives you 100% adhesion.
5. Once you've found a topcoat that works according to your test spot, proceed with coating the box.
6. In the highly unlikely event that you can't find a topcoat that gives you good adhesion on your test spot, it may be less hassel to sand the powder off entirely. However, in 30 years in the business, I've yet to see a powder coating that can't be topcoated sucessfully IF the proper steps and an optimum topcoat are used.
Good luck on your project. others on this forum might appreciate hearing how you get along and what topcoat you found to work for you.
Certain tool boxes may be more difficult than most articles to recoat successfully, as there have been known to be tool box manufacturers that require extreme chemical resistance when specifying their powder. (Others should be a piece of cake to recoat.) Having said that, it should be possible for you to get the job done succesfully, albeit with a little more effort than usual. Here's how I would proceed:
1. Remove the peeling topcoat completely and thoroughly.
2. Go ahead and sand the powder coating surface thoroughly. I'd use something fairly coarse, probably around 120 to 220. Finer is less advisable for what you're trying to do here.
3. Your good candidates for liquid topcoats would include either epoxy-polyamide primers, or a 2K urethane. If you should wind up sanding through the powder to bare metal in a spot or two by accident, then you might lean more in the direction of the epoxy. You should do a small test spot in an inconspicuous place to see how the adhesion is going to be on the particular product you wish to use before you paint the whole thing. This is a critical step- don't skip it! If you have multiple choices, your best chances of sucess will probably be paints with fairly strong solvents, preferably including some ketones like MIBK (methyl isobutyl ketone), MAK (methyl amyl ketone), or (MEK) methyl ethyl ketone, or esters like butyl acetate. I would definitely stay away from single component basecoats as your finish directly over the powder (some do work sucessfully, but your chances of sucess are improved with a two component product, all other things being equal).
4. Let your test spot cure thoroughly (best several days to a week at 70F or above), then take a knife and cut an X through the topcoat. Put some stong tape on it and check the adhesion. A proper choice of topcoat should be one that gives you 100% adhesion.
5. Once you've found a topcoat that works according to your test spot, proceed with coating the box.
6. In the highly unlikely event that you can't find a topcoat that gives you good adhesion on your test spot, it may be less hassel to sand the powder off entirely. However, in 30 years in the business, I've yet to see a powder coating that can't be topcoated sucessfully IF the proper steps and an optimum topcoat are used.
Good luck on your project. others on this forum might appreciate hearing how you get along and what topcoat you found to work for you.
This box is steel and is new so the powder coat is clean and undamaged. It appears to have been scuffed by the previous painter and the paint (not sure what type) was sprayed directly over the powder coat without any kind of primer/sealer. The way this stuff is coming off in huge sheets makes me tend to suspect some kind of improper surface prep but since I really know little about powder coat I have to admit I am kind of worried that the same thing could happen again.
Bob, I will seal with epoxy what grit would you use to sand this with?
Bob, I will seal with epoxy what grit would you use to sand this with?
Powderbill, Thanks for the reply we must have been typing at the same time 
, I will do that test since this may not be as simple as I first thought. I will have to paint this with the Nason single stage urethane since it has already been bought but if sealed with epoxy this should be ok?
, I will do that test since this may not be as simple as I first thought. I will have to paint this with the Nason single stage urethane since it has already been bought but if sealed with epoxy this should be ok?From what you describe, I wonder if it was actually powder coat on it, or a vinyl cladding of some sort? All powders I have seen will readily accept any sort of paint if scuffed properly. If it is indeed something other than powder, then total removal would seem to be the way to go. Hopefully it's not diamond-plate?
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Oldred, an epoxy should work fine over the powder, after preparing the surface as outlined above. The only real incentive for using a 2K urethane, which typically will adhere well to powder, is you'd be able to get by with a single coat of topcoat and still have a weatherable system.
Krehmkej, This appears to be powder coat but the surface is smooth so if I have to strip it then it should not be too bad. I sanded a spot with 180 and shot some epoxy on it and it SEEMS to be adhering well but it has hardly had time to dry so I will wait a day or so and do that test Powderbill suggested.
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