Spraying base coat urethane...Is sealer necessary?

[crowe-t]

Is a sealer necessary or helpful when spraying a plastic body with automotive basecoat urethane?

Duplicolor makes a Primer-Sealer.  Is this good for helping to prevent the basecoat urethane from eating through the primer and plastic?

If sealer is used over primer should more primer be sprayed over the sealer?

[Daddy Mack]

The basecoats i've been using have solvents that behave like lacquer thinner and they will attack model car plastic and certain paints. So ya, i seal. Been using Duplicolor self-etching primer and now Rustoleum makes it as well. Seems like you're supposed to wait an hour before topcoating it with basecoat and has to be done before 24 hours or else you have to sand the self-etching primer for adhesion. I have no experience with D-C Primer-Sealer.

[crowe-t]

2 minutes ago, Daddy Mack said:

The basecoats i've been using have solvents that behave like lacquer thinner and they will attack model car plastic and certain paints. So ya, i seal. Been using Duplicolor self-etching primer and now Rustoleum makes it as well. Seems like you're supposed to wait an hour before topcoating it with basecoat and has to be done before 24 hours or else you have to sand the self-etching primer for adhesion. I have no experience with D-C Primer-Sealer.

Do you spray first spray the plastic with a regular non self-etching primer before spraying the Duplicolor self-etching primer?

I would think that self-etching primer would attack the plastic but I could be wrong.

[Daddy Mack]

I'll put regular primer on beforehand to fill in flaws but by the time i spray self-etching there will be a lot of bare plastic showing after sanding. The self-etching primer has acid in it for etching metal but in my experience it does nothing to model car plastic. Read up on organic vs inorganic acids for explanation.

[64SS350]

I use Duplicolor primer, sometimes sealer, especially if I need a lighter shade. But as long as you have that auto grade barrier, you should be good.

I also thin with lacquer, low end, not even medium.

Just started using base/clear acrylic-enamel on this Stude project, looking forward to seeing the results.

Wouldn't bother with the self etching, no need.

[Ace-Garageguy]

On 4/1/2019 at 10:50 AM, crowe-t said:

...I would think that self-etching primer would attack the plastic but I could be wrong.

 

On 4/1/2019 at 11:00 AM, Daddy Mack said:

... The self-etching primer has acid in it for etching metal but in my experience it does nothing to model car plastic...

Self-etching primer is generally formulated with HOT thinners, and they do indeed craze plastic, especially if shot at all wet.

I've used 'em all extensively, as I always have stock from my real-car work. SEM and Duplicolor will both etch and craze most styrene badly...particularly the bottom-of-the-barrel soft goo kits are made from in China these days. Even non-self-etch automotive primers will wreak havoc with the surface of most kits, if they're shot straight from the can, wet enough to flow out.

The ONLY styrene I've found to be relatively impervious to self-etch is old Johan annuals.

Far as sealer under automotive basecoat goes, there's no hard-and-fast rule, even on REAL cars. In general, the fewer coats of ANYTHING you can shoot to get the desired finish, the better. Sealers are promoted by the paint industry as necessary because they're profit-makers, and they're often simply not needed.

Sealers are most correctly employed when a bleeding or crazing problem occurs, and each case should be treated as a separate event. TEST the base you want to use on the underside or inside OF PARTS FROM THE SPECIFIC KIT YOU'RE WORKING ON.

[Daddy Mack]

3 minutes ago, Ace-Garageguy said:

Duplicolor will both etch and craze most styrene badly

Lolzzzzzzzz!

[Ace-Garageguy]

3 minutes ago, Daddy Mack said:

Lolzzzzzzzz!

You disagree?

[Daddy Mack]

25 minutes ago, Ace-Garageguy said:

You disagree?

Confession Time:  I cannot help myself from being amused after i try something and have first-hand experience with it and then someone, who has never done it, comes along and says i cannot do what i already did.

I'm working on a model car hood right now that is made of styrene plastic and i filled the hole with a piece of styrene from a model car roof that had a similar curvature. The first thing i sprayed onto it was Dupli-Color Self-Etching Primer. And i'm filling in the little pits and what-not with paint-brush dabs of the same primer. There was no reaction from the plastic by the application of wet Dupli-Color Self-Etching Primer. This is a pretty rare hood so i promise you, this is not the first time i have blown self-etching primer on bare styrene.

I gave a clue in a previous reply in this thread. There is a reason why some acids are stored in glass containers and others in plastic.

[Ace-Garageguy]

2 minutes ago, Daddy Mack said:

... and then someone, who has never done it, comes along and says i cannot do what i already did...

I guess you failed to read the part that said I use the stuff EXTENSIVELY. Have for YEARS.

The plastic you're shooting the stuff on is most likely a harder, more solvent-resistant grade than what's commonly available today. Or not. Duplicolor has been changing formulas and packaging frequently lately. I have no idea what exact product YOU have, but I know how the stuff I have works.

I have several models whose surfaces are so degraded by the crazing from Duplicolor products, they are unusable for anything but rust jobs.

I don't talk out my backside, as just about anyone here knows.

[restoman]

Much like everything in the automotive paint world: sometimes you get good results, sometimes you don't.

I've used high build primers, etching primers, primer sealers, sandable primers, etc., from many different manufacturers such as Tamiya, Dupli-color, Rustoleum, etc.

Most times, I've never had a problem, probably more to do with my slow and careful approach learned over 30 years as a professional painter. Sometimes, even with a methodical approach, it all goes to cr*p.

The biggest trick to learn when using automotive anything on styrene, is to take things slow. Full wet coats are a no-no. Not waiting for proper flash times is a no-no. Thick coats are a sure-fire way to botch a job. As stated above, newer styrene is pretty soft, but older stuff can react to solvents too. 

I wish I had a knickel for every time I did something that someone told me couldn't be done. And I wish I had a knickel for every time I told someone how to do something and they went ahead and did it differently and with success. That's how it goes... What works for some, may not work for others.