Enamel paint cracking?

[Taylor]

I have been having so much trouble with painting an enclosure. I did one in white with a clear coat, which came out great the first time.

The second one is yellow. I am using spray enamel. I have painted and scraped and repainted this thing about 5 times for different reasons, but I finally got a decent finish and  then did the clear coat, and the paint cracked. Is there some issue with the yellow and clear not being compatible?

I did not bake the enclosures, which I will in the future, but I haven't been able to find a cheap toaster oven around.

[John Lyons]

The first coat needs to be fully cured before you spray the second (clear) coat.
When the first coat is sprayed over with the clear coat it is re wetted by the solvents
in the clear coat and causing the skin of the first coat to shift which is the cause
of the wrinkle or cracking.
When you do get an oven you can fully dry the paint before you apply the clear coat.
It helps to spray several very light coats of clear.
This is because a light coat will not saturate and react with the first coat as there is
less solvent to break down the first coat and cause it to shift. The solvents gas off sooner...
It helps to use the same brand paints as well so there won't be as much chance
for an adverse reaction since the paints have the same solvents and chemicals in them.

A thoroughly cured base coat and many light coats or clear is the main thing though.

John

[Taylor]

Thanks for those tips.

I decided to use my toaster oven to bake the box, since I don't use it for food much anyway (yeah, I realize I can't use it for food anymore), but my paint actually caught on fire.

I'm using a large-ish box, and even though I put the toaster on a very low heat setting, I guess it was just too close to the element. I have never heard of this happening to anyone, so it didn't occur to me that I needed to worry about it.

So, back to scraping...

Fellow noobs, please learn from my mistake.

[Evad Nomenclature]

Hey Tay.

When you bake the box do it on really low heat.  If you have a problem with the element, I would just heat the toaster up, turn it off and then put it inside.   I bought a new toaster over a few months ago for this purpose from a cheapo discount store.  I never caught anything on fire, but had a tiny bit of smoldering if I put the heat anywhere over like 160-170 degrees F.  I can set it as low as 125F, so I usually put it on minimum and let it sit in there for 10 minutes or so.
The stuff dries pretty quick in the oven.  As you found out though, definitely keep your eye on it lol.
How's the house smelling btw? 

[Taylor]

Hey Tay.

Ha!

When you bake the box do it on really low heat.  If you have a problem with the element, I would just heat the toaster up, turn it off and then put it inside.   I bought a new toaster over a few months ago for this purpose from a cheapo discount store.  I never caught anything on fire, but had a tiny bit of smoldering if I put the heat anywhere over like 160-170 degrees F.  I can set it as low as 125F, so I usually put it on minimum and let it sit in there for 10 minutes or so.
The stuff dries pretty quick in the oven.  As you found out though, definitely keep your eye on it lol.
How's the house smelling btw? 

I actually had it on the "keep warm" setting, below all the demarcated temperatures. The thing is that I'm using a sloped-front box which is 3 inches tall in back - it was right under the element and the paint bubbled up, and as I was groaning about that, it caught fire. Looked kind of like burnt marshmallows. If I was doing this for myself, I might leave it, but being a commission... Maybe I should ask the guy if he likes burnt finishes? Maybe I'll get lucky and he has like a burnt Strat he wants to match or something...

[studiostud]

Looked kind of like burnt marshmallows.

Hey... if it looks cool, roll with it!   

[frequencycentral]

I actually had it on the "keep warm" setting, below all the demarcated temperatures. The thing is that I'm using a sloped-front box which is 3 inches tall in back - it was right under the element and the paint bubbled up, and as I was groaning about that, it caught fire. Looked kind of like burnt marshmallows. If I was doing this for myself, I might leave it, but being a commission... Maybe I should ask the guy if he likes burnt finishes? Maybe I'll get lucky and he has like a burnt Strat he wants to match or something...

I sympathise with you plight, I've never had any luck with paint and clearcoat. But this post made me choke on my coffee............................

[Taylor]

Glad my pain is so funny to you.      Haha.

I've since switched to acrylic paint with epoxy resin topcoat - way less of a headache than spraypaint, and it looks great and is super-strong.

[m-theory]

I tried baking a few times, and got horrible lifting issues each time.  I honestly don't have a clue how some people are able to make that work, but I just gave up on it. 

Here's what I've found, in painting many, many boxes over the past few years...

The "recoat times" that are printed on the back of the can mean something.  They'll generally say something like "if not recoating within 1 hour, wait 7 days," or something along that line.  This is because, after that first hour (or 3, or whatever), the "window" slams shut, and the substrate becomes sensitive to fresh solvents, and will lift/wrinkle. 

After 7 days, the solvents in that painted film have fully evaporated, and thus aren't as likely to be attacked by fresh solvents.  It's important to recognize that, unless you're adding a hardener, which you aren't with spray bombs, the paint is NEVER actually "cured."  The term "cured" refers to a chemical conversion process that occurs when you add an isocyanate hardener.  The film undergoes a molecular change, and becomes a form of plastic.  That doesn't happen with spray bomb.  The best you get is "fully dry," which is what happens when the solvents have all evaporated from within.  In essence, any paint film that isn't catalyzed can be "re-wet" with solvents, even years later.  (Don't believe me?  Soak a rag with lacquer thinner and rub a spot on the hood of your '65 Chevelle and see what happens!) 

Meanwhile, what happens is that the surface skins over, but solvents underneath that film remain live for several days.  When you spray something over a film that has skinned over, it penetrates the outer film and reacts with the solvents that remain underneath, thus causing the dreaded "wrinkle." 

When you get a lift like this, you MUST completely strip and start over.  Again, we're not dealing with catalyzed products here, so we cannot "lock" sensitive substrates down with catalyzed primers.  Once that outer film has been compomised, it will be VERY sensitive to additional lifting, and you're best off just stripping it completely and starting over. 

The key to avoiding this lifting without having to wait 7 days between recoating, is to finish your application process in that 1-2 hour window.  I've painted more boxes than I can count in the following manner, and haven't had a lifting problem since I started doing this: 

I start by rough sanding the box with 220g.  Then, I acid etch.  This is messy and a PITA, but it does work to get excellent initial adhesion.  Aluminum is a terrible substrate to get paint to stick to on its own.  You can skip the acid etch process by using a self etching aerosol primer.  That stuff only needs about 10-15 minutes to "dry," at which time, you can apply a fill/sandable primer.  When that's dried sufficiently to sand, wet sand w/600g paper. 

Dry the parts, set them back up on whatever it is you set them on to paint, and let the surrounding air remove whatever moisture remains.  In 10-15 minutes, start applying color coats, with the first 2 coats being very light.  Wait about 5 minutes between these light coats.  You don't need coverage on these coats, all you want is to get a "tack film" set up.  The third coat can be a medium coat that will give you your coverage.  Depending on the opacity of the color, you may need more than a second medium coat.  Wait about 10 minutes between the coverage coats.

Let this set up for a good 20-30 minutes or so.  Again, you have that 1 hour window to work with.  Before that window slams shut, you'll want to apply a clear, if you're using a spray bomb clear.  If you don't get the clear on within that hour, you'll be waiting 7 days. 

I personally can't stand those spray clearcoats.  I've tried different brands, both lacquer and polyurethane, and they all work about the same.  They're thin, so you need many, many coats if you intend to sand and buff afterwards, and they dry "dull."  They also aren't that good at providing protection. 

I much prefer to use the 2-part acrylic epoxy clear that you can buy at a hardware store, in the "varnish" section.  One coat of this stuff (which pours as a liquid) is the equivalent of 50 coats of standard varnish.  It lays flat as glass, dries to a porcelin-hard finish, and looks wet when cured.  It's a PITA to work with, because it's messy, requires strict adherance to application guidelines, and takes about 30 hours or so to dry, and probably a good week to fully cure, but it's awfully tough to beat the end results you get with it. 

[Toney]

You started with etch primer, right?
The only way with aluminium....