Spray Painting Issues

BillDL

I have a carved top solidbody electric guitar on which I am respraying the top using nitrocellulose aerosol lacquers. The old finish had been scraped and sanded off where in places it was through to the wood and in other places it was back to the clear sanding sealer or initial base coat used by the manufacturer.  I sanded it down to 240 or 320 grit, applied a couple of coats of clear sanding sealer, and sanded that down to 240 or 320.  It was perfect and was absolutely wiped free of any potential contaminents ready for a grey primer.

I intend spraying a metallic gold lacquer over the primer, then tinted clear to make the gold more subdued and mellow, then some fine overspray of clear with a black tint, then blend in a black edgeburst before sealing it with clear top coat.  I did the headstock this way and it has come out exactly as I wanted.

About 3 or 4 minutes after applying a fine spray of the grey primer it started to separate into fine ragged veins that spread and resemble dozens of little weaving rivers from a high up satellite view of the earth.  They separated back to the sanding sealer to show the wood colour like stretch marks and there's no distinct pattern to it.  Some parts are hardly affected while other parts have lots of these.  I let the primer dry and then tried to mist on some more primer in the worst areas in the hope it would fill in the valleys and I could sand it back, but that too cracked / crazed within a few minutes as it began to dry, and in new and different areas. 

I was doing this outside on an absolutely dry and clear and fairly still day that is neither particularly warm nor chilly and I sprayed it in the shade.  I had taken the body and the can of lacquer from inside my house outside about 90 minutes before I sprayed it to acclimatise them to the conditions.  I wasn't heavy handed with the primer and didn't over-wet the surface. I've only ever had this happen once before on a very small patch on a guitar body and I think (although I can't be sure) that it may have been sticky residue from a tack cloth.  It's never happened over a wide area like this and in such an inconsistent way.

This particular nitro sanding sealer and primer were bought very recently from the same large online retailer, but I'm not going to name them as I don't want there to be any inference that I'm blaming the lacquer.  I shook the can of primer for a long time prior to spraying.  As far as I'm concerned I mitigated any possibilities of it going wrong and I can't understand what has caused this.

The only difference between the headstock that came out perfectly and the body that's gone wonky is that the headstock was sanded back through the layers of existing lacquer until I was just starting to see the wood colour below showing.  I used the same primer on that, but didn't spray sanding sealer first because it wasn't needed.

Of the possibilities I have wondered the following:

• Maybe a bit too chilly outside for spraying?  Even though I was perfectly comfortable in a T-Shirt it wasn't what you would describe as warm.
• Sanded the sanding sealer down with too fine a grade of paper where there wasn't enough of a key for the primer?
• Some residue on the sanded down sanding sealer that's making it separate?  I wiped it with a dry soft cloth from an old T-Shirt.  I suppose it's remotely possible that there may have been some trace of laundry detergent or fabric softener on the fabric.

Any thoughts about what might have caused this, or has anybody else experienced this issue? 

m_c

Sounds like the primer is reacting with the lower layers.

BillDL

Thanks @m_c.   I am leaning towards this possibility myself, given that there was no sanding sealer under the primer coat on the headstock that has come out perfectly, but it's weird because I know that when it comes to sanding this botched primer coat back off it will have adhered perfectly well without cracking in some of the areas but not others.

GoldenEraGuitars

If the grey primer is acrylic, that might be the issue.

BillDL

Good point, and thanks @GoldenEraGuitars.   I just looked at the can of primer AND the Sanding Sealer to be be doubly sure.  The descriptions on the website for each product, the labels stuck to the cans, and also printed on the cans, is of "Nitrocellulose Guitar Paint / Gray primer" and  "Nitrocellulose Guitar Paint /  Clear Sanding sealer".  This is the first time I have used the sanding sealer and primer from this particular company because I wanted to try grey rather than the white primer that is sold by the retailer I have previously bought lacquers from several times.  The retailer I bought from has a high turnover and a good reputation for selling quality lacquer, but of course that doesn't necessarily mean that there is not an incompatibility between the sanding sealer and the primer.

BillDL

Just an update, if anybody is interested.  It's definitely the clear sanding sealer that is the problematic / incompatible product.

The clear nitrocellulose sanding sealer and grey primer were bought from the same retailer, and it was my first time buying aerosol lacquer from them.  After applying a couple of coats of sanding sealer, flattening it well, and ensuring that the surface was immaculate, the primer immediately began to split into tiny crazed cracks like the tributaries of a river as seen from a satellite almost immediately I started spraying it on. 

I sanded all the grey primer off right back to the sanding sealer then tried some white primer from another retailer.  The same thing happened.  I've used that particular primer over the same vendors own sanding sealer several times before and it was perfectly compatible.

I sanded the body right back to the wood, losing some of the body contouring in the process, which is annoying.  This time I sprayed it with sanding sealer from the retailer I've used before several times, and again allowed it to dry and flattened and cleaned it well.  When I sprayed on the grey primer from the other retailer (i.e. the one from which I also bought the sanding sealer at the same time as the grey primer) it has come out perfectly with no reaction or separation of the primer.

All I can deduce from my steps is that the clear sanding sealer from the retailer I had never bought it from before does not play well with nitrocellulose primers from that same retailer and another.  I am going to email the retailer about this for their information and they can carry out their own tests if they are interested enough, but I won't buy their lacquer again.  I'll just stick with the one I know that seems to be the most popular retailer with members of this forum.