rough patterns in varnish.

Hi, I'm trying to refinish a piece of wood here, and I've painted it black, and am now varnishing it with a gloss varnish. For some reason in the last few layers of varnish, I am getting strange lumps in it. It's almost like the sanding is making air bubbles to appear under the varnish when spraying it on. I am using a 400 grit sandpaper between layers, and getting those issues. The surface feels perfectly flat before varnishing, but those lumps keep appearing. Does anyone know why? (Hopefully you can see it in the image.)

Attachment 330159

Maybe the paint and the varnish are incompatible?

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Originally Posted by Toymaker Len

Maybe the paint and the varnish are incompatible?

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Prior to this, I decided to sand it back to the wood and repaint it again, after having this same issue with varnish on varnish. Both layers were the same, so I really don't know what is going on.

Without knowing a thing about your spray setup, I'd suggest:

More thinners in the varnish.
Higher air pressure.
Different tip on the spray gun.
Slower thinners.
Use a gravity fed gun, not a suction gun.

To me, it looks like the paint might be almost cobwebbing as it comes out of the gun.

Other things to try:

Clean the gun, check the seals and airline pressure.
Make sure the air compressor is drained and you have a filter and drier on the air line.
Make sure the varnish is filtered when you pour it into the gun.

Also check that your flash-off time is correct for the varnish, and also do some test sprays of the varnish on some MDF to see if it's a surface related effect. (if it only happens after a few coats of varnish, then you know it's something to do with technique, not the surface).

EDIT - here's a handy guide (PDF file)... http://www.martinsenour-autopaint.co...otingGuide.pdf

I had the same problem not long back with clear. I was told it was called fisheyes and that I would keep getting it unless I put in on at night or when it was cooler. It doesn't get that cool here and I had no intention of doing it at night with bugs etc.

In the end thinned it right out and used multiple coats earlier in the morning. Still finished up with some but learned to live with it.

Ross

Edit: I brushed it on. Hope this helps as it sounds like what I had.

Best thing for fisheyes is proper* cleaning with wax and grease remover, and if there are still problems after that, adding some anti-silicone additive to the paint before spraying.

It sound like what you had (if it lessened with a drop in temperature) was solvent bubbles - the top of the paint skins over too quickly and the solvent underneath hasn't evaporated, so it blasts a little hole through the skin on the paint. A slower reducer is the typical correction.

*Proper meaning you get a clean rag, pour some wax and grease remover on it (without the container touching the rag), wipe a section, discard the rag, get a new rag, pour some wax and grease remover on it, wipe, discard, new rag, pour, wipe, discard, and so on and not re-using the same rag, as that just spreads it around.

When you say varnish...what exactly.

What are you using to thin, said varnish.

If it is a turps bassed polyU varnish, I have had better results thinning with white spirit rather than turps.....same but less oily.

Remember most retail varnishes are not designed for spraying and they don't tolerete thining too well.....and they certainly do not perform like laquers.

As splinter said.
get realy fussy with your hygeen...have you got paint filters from your auto refinish supplier....always strain when filling your pot and after thinning.

If you have used that gun with another finish type, clean it realy well....make sure you disnmantle the nozzle/needle system and you scrub it out in there.

I run a 3 bath system

If you are working with film forming finishes there is little point sanding further than 180.

with polyU it may be worth letting it go off properly ( like 3 days) if you are sanding between coats.

after sanding do you wipe down with a clean lint free damp rag...and then another realy clean one.

Oh make sure your atomisation is good and lay up the lightest fully wetted coat you can.

watch your temperature and humidity, extreemes of either will ensure problems.

I have a thermometer and a hygrometer in the shed...if it looks bad I wont even open a tin.

OH if you want paint compatable with polyU varnish.....paving paint is polyU and with a bit of skill you will get a good durable finish out of it.

hope ths helps.
cheers