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bubbles in lacquer


john
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3 days ago i lacquered the body of my bass using lechler 2k acryllic lacquer(Acrifan). The body was sanded with 400. I first applied a thin coat , then applied 3 heavy coats(5 min between coats). after the third coat or so, i noticed little bubbles appearing in the lacquer, so i waited to see if they would disapear but they did not.

I continued lacquering as i knew i would be sanding it back anyway.

So i sanded it back with 400 yesterday and the bubbles have gone.

but the question remains, What cause the bubbles?

bear in mind , i ran out of hardner so i used an old tin of hardner that had been lying on the shelf for probably a year or so.

any ideas?

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John,

I think it might be this (shameless copy from reranch.com):

"After spraying the first few lacquer coats I've noticed hundreds of small pin holes in the finish...."

Pin holes can be caused by a number of factors. The most common are contamination of the wood by wax or silicon (Armor All used on the case or plastic parts can be a source of this type of contamination), temperature changes during the spraying /drying cycle and moisture in the wood. Wood contamination can be sanded out in most cases. Wiping with naphtha will also help in removing contamination. Temperature can also play a role in the formation of pinholes. If a guitar that has been in a cool environment then sprayed (as might be the case if you are spraying in an unheated garage) and after spraying moved into a warm environment to dry, air bubbles may form as the wood warms and push their way through the lacquer leaving pinholes. The last frequent cause (and probably most common) is moisture in the wood. Water can get into the wood through washing stripper from the wood, wet sanding primer or sealer coats or not allowing a water based filler or water based dye enough time to dry.

The solution? Sanding and wiping with naphtha should remove wood contamination and not subjecting the wood to major temperature changes during the spraying and drying process will prevent pin holes caused by bubbles (note that placing newly finished wood in the sun to dry is a guaranteed way to cause pin holes). Allowing the wood sufficient drying time after wetting will most likely eliminate moisture related pin holes. A sure way to prevent pin holes before spraying the lacquer coats is to seal the wood with a clear sand and sealer on translucent finishes and sand and sealer and/or a white pigmented shellac on opaque finishes. If you didn't seal the wood and now have pinholes you may be able to drop fill them with unthinned lacquer. The lacquer will over power the cause of the hole and allow subsequent sprayed coats to flow over the holes. After spraying about two coats the "bumps" from the filling can be sanded flat. Drop filling will work on sectional pinholes but if the holes are numerous and over a large area, starting over (this time either correcting the problem and/or sealing the wood before spraying the lacquer) may be the best solution."

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hmm , must be moisture in the wood, come to think of it i remember taking most of the dust off it with a slightly damp cloth..... must use a tack rag next time.

Thanks for your help coen.

hopefully i`ll have it done by the weekend so i can post pictures!

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The other issue with laquer is, waiting 5 minutes is usually not a great choice as it will flash and shrink funny at that point.

When I was spraying laquer, I would spray continually, or, if I had to stop, wait at least 24 hours before recoating.

What happened that made me quit, was I sprayed 3 bodies at once, using the 5 minutes between coats method, the next day, the clear on all 3 bodies had cracked, I had to completely strip them. What happens is, the clear on the final coat, flashes on the outer surface first, and as it cures through, it shrinks and cracks the top coat. When you allow each previous coat to flash off, it starts to shrink. When you spray on a new coat, it melts into the shrinking surface, the top will always flash first (create a skin). If you spray it continually, you are letting it become one complete coat.

The other thing I've had happen, is sometimes, for whatever reason, my newly sprayed coat doesn't melt properly into the previous, and when you sand it out, and polish, you end up seeing lines where you sand through each previous layer.

I am a true believer in that spraying laquer is an art form, and one which I don't have time to pursue.

Oddly enough, I've never had a problem with it on acoustic instruments, only solid body. Weird

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that can also lead to another problem, thick coats will flash on the outside first and shrink faster than under it, causing cracking, laquer is best applied in many thin coats. 25 coats of laquer on a guitar is not unheard of. One coat of a Poly is equivalent to 2.5 coats of laquer, I use 10 to 12 coats of poly when I spray, which is 25+ of laquer.

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