The Stovebolt Site Forums The Shop Area Paint and Body Shop Primer/"Grainy" pattern

Hi guys,

Yesterday I was spraying 2K high-build urethane primer (mixed 3:1 as called for on spec sheet), using a Devilbiss HVLP gun, 1.8 tip; pressure set at 50 lbs at the tank, 30 at the gun inlet (and I presume about 10 lbs at the nozzle). Coverage was good, paint went on fine without runs, etc. But on several of the pieces I noticed a bit of a "grainy" finish--looks a little bit like leather, rather than glass, if you understand what I'm trying to describe. Since this primer will be boarded/sanded after I repair of few dings and scratches, etc., and will be followed by another coat of primer, then a sealer coat, I'm not too concerned at this point with the smoothness (or lack of smoothness) of this finish.

BUT, I want to make sure that the next coat of primer and sealer are smooth like glass. What did I do wrong? Too much pressure? Not enough paint? Too close with the gun? Any help or advice will be greatly appreciated.

Thanks in advance!

JM

Did you read THIS THREAD?

You'll have to thin it down. Simple.

This brings up an interesting question: What do you use to thin urethane primer?

One would guess lacquer thinner because it's what you use for clean up, but can you you use it to thin without adversely affecting the curing process?

Isn't it a bit odd that the spec sheet tells the reduction ratio but fails to mention with what.
I guess I would use Lac. thinner if it didn't say cause once the thinner evaporates out your left with the primer.

Sounds like you need to thin out the primer some more. The primer hits the surface but because its so thick there is no flow out, hence the rough finish.

It also may be a good idea to shoot something else before you prime your project just to check how the gun is putting out and how the paint is laying down.
I have a multi-colored trash barrel which I use for that purpose, because unless I'm using the same primer,same gun, things might be different. Plus I use it to adjust the size of the fan the guns putting out which I want a different size depending on the size of what I'm about to spray.

Dan

Nooooo!... Don't use lacquer thinner. Use a urethane reducer (non reactive).

Most high build primers are supposed to go on kinda thick and grainy, especially if it calls for no reducer. That's why its called high build. They are meant to be sanded and blocked out. once its final sanded then it should be smooth as glass.

If you want a primer that lays down a little smoother, find out if there is a reducer available for that primer, or switch to a primer that uses reducer.

I think 30 psi at the gun for an HVLP gun is a little high which could be part of your problem. Try 22 to 25 psi at the gun (measured with the trigger pressed) and see how that works. The technical data sheet for your primer should say what to reduce it with. If not, ask the vendor where you bought it. I agree with no using lacquer thinner. Joe

I was at my PPG dealer yesterday and asked about thinner for the Nason Select Prime 2K I use and they told me there wasn't any, but that's just for that kind and I didn't ask about others. Like Jeffl says, primer is supposed to be sanded and I've never heard of not sanding it prior to painting. All of my primed surfaces are sanded with 400 grit wet... and when done, it's smooth as glass.

On a related issue, I always thought that sealer was only recommended if you were painting over anything other than a completely primed area like a repair where part of the panel is primer and the rest a color. Isn't the purpose of sealer to prevent bleed through of anything that could contaminate the finish?

Sealer is used for a uniform base and seals sand scratches, and it is a wet on wet application.

In our shop we use sealer on all new parts that have been sanded and over bodywork that has been primed and sanded. If color is going on an entire panel, or if its an overall paint job, one coat of sealer goes on everything. Once it flashes off you go straight to color.

I'm not trying to be difficult, just trying to understand.

I still don't understand why you need sealer in all applications. If you take a part and prime it, it has a uniform base, and by this I assume you mean a uniform base color... correct?.

What grit sandpaper are you using that you have to seal sand scratches? I thought the standard procedure was to wet sand with 400 which leaves a VERY smooth finish.

I'm confused.

It is confusing. You don't always need to use sealer in all applications. A lot of silvers & gray metallics will pickup sand scratches easily, even 400 grit. Sometimes it takes a lot of paint to cover those scratches, the sealer will eliminate that. Sometimes the paint just lays down better with sealer. We sand everything with 400 dry, although wet sanding is the proper way to go, dry sanding is faster & a little less messier. I think you will find more wet sanding done in a restoration shop than in a production shop, & I think more production shops use sealers than restoration shops.

On uniform base, yes you are correct, but that's not always the case. Say your'e doing an overall & half the truck is in primer & the rest is just paint sanded with 400. This is where the sealer comes in. One coat over everything, let it flash, then color. Now you can have even color coverage instead of one area covering faster than another. There are a lot of colors that dont like to cover too good.

Hope this helps

Thanks Jeff, a lot has changed since the 70s and painting cars is a whole new world.

I understand now and thank you for the detailed answer

Thanks, guys, for the responses and good discussion. My tech sheet definitely says 3:1 ratio of paint to catalyst (called "primer hardener"), with no reducer. This is the direction for priming. It then has another direction that says, for sealing the ratio becomes 3:1:1, i.e., 3 parts paint, 1 part catalyst (hardener), 1 part reducer. This, as Jeff says above, is to be sprayed wet on wet, which I understand to mean that once it flashes, you follow up with the color coats.

Following further reading and instruction, my intention is now to touch up any final body work (dings, scratches, etc.) with light body filler (is glazing putty acceptable here?), then sand everything again with 400; then apply one more coat of primer. Sand again, then seal with one coat in the 3:1:1 ratio (mentioned above), then apply the color coats. This is the technique I've read about in several sources. Does that sound like it's on the right track?

Thanks,

Jim

Yep, that sounds right. What kind of primer are you using? It sounds a lot like the primer that we use.
We are using R-M DP-26 powerfill primer, the mixing ratio is 4:1 with no reducer. It sands really nice, & fills awesome. You can actually fill dents with it.It can also be used as a sealer. To be used as a sealer it gets tinted with R-M single stage Uno with a ratio of 3:1:1:1. It lays down nice & flat.

Hey Jeff, thanks for the info on that primer. I'm not familar with it so I guess maybe I should have held back on my comments.
Being in the trade keeps you up on the stuff that hits the market.
I'm not in the trade every day anymore, so it looks like I'll have to do some checking on stuff. I understand what you're saying about a production shop vs any other. You do what works to get it out the door the best, plus fast

Dan