Pinholes in poly

[Mitchell898]

Hi everyone, I'm finishing my first guitar and i've run into an issue with the final clear coat. As you can see in the photo it's a mahogany body with a dyed maple top. The maple has dye, vinyl sealer then top coat. The rest of the body including the raw maple edge and mahogany came out great but the stained area has lots of little dimples (pinholes may not be the right word) hopefully the photo captures it well enough to see whats going on. The clear is Spraymax 2k, it's been drying for about 6 hours. My current plan is to sand the whole body as level as I can once 24 hrs has passed and recoat hoping the dimples fill in. Any other suggestions would be greatly appreciated. 

 

[killemall8]

did you seal it first or is that on raw wood?

[Bizman62]

Either the vinyl sealer hasn't done the job properly or your clear coat has either reacted with uncured sealer or the spraying has been inaccurate. That looks like burst bubbles or something like that, a bit rougher than the orange peel often seen.

If that was your first coat of final clear coat, it's common practice to sand it matte and respray. Getting the bottoms of the dimples matte is important as they would otherwise shine like stars under the next coat. Lightly level with some 400-600 wet paper and a block to see how bad the dimples are, followed by an abrasive felt to address the bottoms if needed. Then respray. The next coat should then be better. You may still have to lightly sand even the final coat matte and then go through the grits to finer and finer abrasives up to very fine polishing pastes for the ultimate mirror like gloss. A sprayed surface is glossy but it's not level as you noticed.

[ScottR]

This is good advice, except I don't think you need to get the bottom of the dimples matte. The surface gloss you see is going to be based on the surface your last coat.  You can knock the tops off the orange peel and spray more coats hoping to fill in the dimples. If they don't fill in then you may very well need to sand to the bottom to get them to fill.

Now with finish leveling and polishing that becomes more important. If you do not level to the lowest level--the bottom of the dimples will shine like stars. Even if you polish the surrounding surface to the same gloss level, the difference in surface angle in the dimple will reflect the light back at a different angle., which can be visible.

You want to spray and level enough coats to insure you final leveling is ....well, level.

SR

[Drak]

As far as I know, the 2k aerosols are going to be much more similar to an epoxy finish than any film finish (i.e. shellac, lacquer). Film finishes that melt into each other are called 'brittle' finishes, where crosslinking A/B finishes are called 'tough' finishes (think like a tire tread here). They may both look glossy, but as a finished, dried product, they have nearly nothing in common with each other.

Film finishes via their brittle nature, allow you to 'finish the finish' via sanding to finer and finer grits, then final buff and polish. 2-part crosslinkers with resins and hardening agents don't respond to 'finishing the finish' without expensive pedestal buffing stands, very high heat, the right products, and the educational skill sets on how to use all of that equipment. 99% of which 2k aerosol users don't have access to...if they did, they wouldn't be using aerosol spray cans in the first place.

Imagine trying to polish a tire tread to a high gloss finish using finer and finer grits and you get the picture of 2k products. I believe what they do is actually 'melt' the top with the pedestal buffing stand, very high heat, and the correct products and it takes experience to know how to do that.

This is the drawback to using the 2k products, your final coat must be perfect, because once you shoot it, you're done. There is no 'finishing the finish' with 2k's usually.

Yes, of course you can level sand prior coats to achieve a dead-flat level surface on which to spray that final coat on to. But once that final coat is sprayed down, its game over and done. There is no 'finishing the finish' on a 2k product w/o all the correct (and very expensive) finishing equipment.

I believe I've read with those 2k aerosols to heat them up first in warm to semi-hot water, which makes perfect sense because that's exactly what you do when using 2-part epoxy, at least I always have. Which shows you the 2k's are much more like an epoxy finish, and it helps to wrap your mind around that and get away from thinking its any kind of lacquer-ish product, because its not 'that', at all.

 

And I would agree on some sort of chemical interaction between the vinyl sealer and the 2k, as that seems to be the only difference between the top and the rest of the guitar. Its possible the 2k product, while it was chemically reacting (curing) attacked the vinyl sealer.

The old finishing rule is never apply a harder finish over a softer finish. You can go the other way around and be OK tho.

[Bizman62]

30 minutes ago, Drak said:

2-part crosslinkers with resins and hardening agents don't respond to 'finishing the finish' without expensive pedestal buffing stands, very high heat, the right products, and the educational skill sets on how to use all of that equipment. 99% of which 2k aerosol users don't have access to...i

This one has automotive 2k (the type suitable to use with metal flakes) sprayed with a €15 spray gun. It's basically hand sanded up to 6000 grit (I may have used a random orbital sander for some grits, moist but not wet) and polished with a swirl remover fluid (the finest of a three bottle system) and finally waxed with Simoniz Classic "car and furniture" wax. I could almost read this page on it, the photo is not that great.

[Drak]

Beautiful!

Should I delete?

I will add, about 20 years ago I did buy a gallon of pre-cat lacquer and its accompanying agent and shot a few Coco-Bolo guitars with it. Which is why I bought it in the first place, as Coco-Bolo oils seep up into film finishes and darken them, like a tint, which ticked me off. I needed a finish that didn't melt into previous coats and the sealer coat would 'stop' any CB oils from seeping, it 'locks' them up.

Using the same methods that I've always used, I could never get them to shine and gloss up sitting side-by-side with my lacquer jobs. They had a slightly 'matte' look to them in comparison and my finishing research reading materials backed up my own experiences. That information is what I posted here, I didn't just 'make it up'.

But pictures don't lie, that looks great from here.

[Bizman62]

11 hours ago, Drak said:

Should I delete?

No. There's 2k and there's 2k. The ones we're using are intended for industrial use, a proper spray cabin being a must. Professional luthiers in Finland seem to use the IL500 by Akzo Nobel, the local car paint shop sold the other brand after hearing we have a professional grade spray cabin and a teacher knowing the health regulations. Both are glass clear.

As you said, 2k doesn't melt to the previous layers. The trick is to first apply a thin layer and sand it matte for a level surface - that serves as pore filler as well. The final layer is shot as wet on wet, allowing the 2k to harden for about half an hour between coats to prevent runoffs. As you know, unlike lacquer 2k doesn't dry starting on the surface, it hardens chemically instead. Thus you can let it solidify a bit before applying more for a uniform single thick layer which can be sanded and polished to the level shown in my photo.

[Mitchell898]

Thanks for the replies! I ended up leveling as much as I could and sprayed 2 more coats. The dimples were almost completely gone, 1 or 2 were still visible. I wasn't able to matte the low points and they're not visible at all in the final product. There were a couple imperfections from small knots in the maple but thats a seperate issue. Overall I'm pleased with the result. Here it is.

 

[Bizman62]

Yummy!

[ScottR]

Yep, that's really nice!

SR