Official Luthiers Forum!

I'm on about coat #10 of a nitro finish. This would be my second session with some sanding after 6 coats. All the previous coats have gone on well enough but the last coat I did yesterday went on with what appears to be quite a few air bubbles that made for a less than perfect coat. I left it over night and sanded it back this morning. They were fairly easy to remove. Put down another coat and it seems to be doing the same thing. My gun is clean. I'm a little perplexed. Anybody have any thoughts why all of a sudden this might happen when I haven't done anything different? It's a bit of a head scratcher. I tried to get a photo but it's a little hard to capture.



Thanks for any help.

Cheers,
Danny

I can't see squat in your picture, but I'm guessing what you are experiencing is solvent popping which is caused by not waiting long enough between spraying sessions...

I find using a spraying schedule of 5 coats maximum in one day and waiting a week between sessions to be appropriate...for one thing 5 coats in a day (with one hour between coats) is a lot of material to be laying on and one must wait a period of time to allow the solvents to flash off fully before laying down 5 more coats...the period of one hour between coats allows for minimal flashing off of solvents (i.e. drying time)...this advice is of course for air dry (non-catalyzed) finishes...

I also usually spray my first coat after the week long waiting period a little thinner as I believe that the extra solvents allow for better bonding to the previous coats...

if you find your self having to use some sort of retarder because of humidity or temperature issues than waiting longer than 1 week might be needed

A draft can cause this, it causes the surface to dry faster than the body causing the solvent pops Mike mentioned.To fast of a thinner can have a similar effect. Another culprit is moisture droplets in the air supply. If your separator is right at the compressor and you have a good length of hose, condensation can form in the hose and will cause this type of problem in the later coats.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Thanks gents. I will give it a bit of a wait Mike. I never have had to in the past and it did have a couple days to cure before the second session. It was hard to get a good photo of the problem.

Could be moisture Brian. I have 2 traps, one right at the gun and then about 6' of hose to the gun. Come to think of it I haven't checked the traps since I recently sprayed a kitchen. Thanks for the reminder. There is a bit of a breeze blowing past. I have a 24" fan doing the sucking but it's never been an issue.

Appreciate the help.

Cheers,
Danny

you didn't specify in your original post, but I find myself wondering just when these bubbles appeared..if they appeared immediately or within seconds then solvent popping is most likely not the cause...I had this issue once with a brand new Binks Mach1-SL HVLP system being used with a pressure pot...long story short it turned out that the brand new gun had failing internal seals which was allowing air to be mixed into the lacquer before the atomization stage...the result was immediate foaming/bubbling of the product upon hitting the wood...

I would say gun problems.
Is it a good gun?
Settings right?
Maybe lac not thinned enough?
Are they bubbles, or fish-eyes?

It's immediate Mike. Not a lot. Maybe 5-8 bubbles per 4 sq. inches. Just enough to p*## a person off when you're trying to lay down some final coats.



It's a decent gun Alan. A Walcom Ego. It's dedicated to instruments and this would be the 4th or 5th since it was new. Like I say it was and has shot very well and then 4th coat of the day it did that. Same mixed batch. Had the settings dialled in and I find about 20% reduced seems to works well. They are bubbles, about the size if a pin head. It came with a replacement seal kit. Maybe that's a bad oman. I guess I'll start with making sure there's no moisture. Otherwise It might be a seal.

Thanks Again,
Danny

Finishing!!!!!!!!!!!!!!!!!!

I certainly don't know about other guns, but both of the Binks (Mach1 HVLP and 2000 conventional) guns I have need to be lubricated every now and again, and one of the points of lubrication is the packing the needle slides through...is your gun similar? your spray gun supplier should have the proper lubrication available...

all seals will eventually fail and that is not an indication of a 'cheap' gun...hopefully a bit of lubing of the packing is all you will need, otherwise replace the seals...

I agree with Mike_P. Was just getting ready to write something similar when I read his opinion. Check the packing and seals or replace and lube the gun without silicon.

Hutch

_________________
"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

Chuck my friend, I'm a frequent luber.(Ooooh, that doesn't sound right). I use mineral oil when ever I take it apart to clean. Got to work a real job today. It's going to have to wait. Thanks for the help.

Like your new quote thingy. Chet's one of my heros.

Cheerio,
Danny