bubbles in Hard Shellac

I've never used Hard Shellac before, so can someone tell me what I'm doing wrong? The wood is Victorian Ash.
The project is the top of a boys desk, so I needed a hard surface. I wiped on 2 coats of 3 parts Industrial Methylate Spirits: 1 part HS. Then wiped on 4 coats 1 part IMS: 1 part HS. Everything seems fine after an hour, then a few hours later the surface is very rough with tiny bubbles. I have tried light sanding 400 grit, I've tried thinning to 8 parts IMF 1part HW, I've tried brushing a coat on. Still get bubbles. It seems to me as though the top coats over, then traps gas from underlying drying shellac. If so is there any way to slow the drying time?
As I said I've never use HS before. I would usually use wipe on poly.
If all else fails, can I now sand and apply WOP or do I have to wait 20 day for the cross linking to occur?
Thanks in advance for any ideas,
Alex

I am sure that Neil will be along soon to help.

I have seen a somewhat similar thing with regular poly and with resin. With the poly I had varnished a piece of timber in the sun on a pleasant afternoon and shortly after finishing noticed tiny little bubbles forming then bursting in the poly. I am assuming that the sun was warming the wood and the air in the pores of the wood was expanding and bubbling out. Have seen the same thing with resin and in that case the bubbles remained suspended in the resin. If I am worried about this happening I will apply resin etc later in the day when it has started to cool so the air in the wood is contracting and sucking the finish into the wood. The finish /resin will have hardened by the following day when temperatures start to rise.

Don't know if this helps you Alex.

I've never used Hard Shellac before, so can someone tell me what I'm doing wrong? The wood is Victorian Ash.
The project is the top of a boys desk, so I needed a hard surface. I wiped on 2 coats of 3 parts Industrial Methylate Spirits: 1 part HS. Then wiped on 4 coats 1 part IMS: 1 part HS.

Few basic questions first:

• Was it new timber or had it an old surface coating been stripped back?
• If stripped was the stripper neutralised and washed down with copious amounts of metho?
• How fine did you sand?
• What did you use to wipe the coats o with?
• How long between coats?
• Was the application done inside or outside?
• Was it hot or cold?

Everything seems fine after an hour, then a few hours later the surface is very rough with tiny bubbles. I have tried light sanding 400 grit, I've tried thinning to 8 parts IMF 1part HW, I've tried brushing a coat on. Still get bubbles. It seems to me as though the top coats over, then traps gas from underlying drying shellac. If so is there any way to slow the drying time?
As I said I've never use HS before. I would usually use wipe on poly.
If all else fails, can I now sand and apply WOP or do I have to wait 20 day for the cross linking to occur?
Thanks in advance for any ideas,
Alex

thanks for replying
it was new timber
sanded to 320 grit. But I did not use a grain filler. Could this be my problem? I thought that a couple of coats of HS would fill the grain.
I sanded between coats with 400 grit to remove bubbles.
wiped on with a rubber, several different covers, firstly a knitted fabric then a cotton sheet fabric probably cotton synthetic blend. same problem with all covers. then tried brushing. same problem
4 hours between coats or overnight.
application done indoors
the temperature was warm. about 25 to 30 degrees
Alex

First problem; will most likely be the timber. Vic Ash which can be very open grained so if shellac or most other surface coats are applied to the open grain especially in warm conditions you may get bubbles of expanding air trying to break through the surface.

Second problem; will be the sanding 320 grit (often sold as fine) is fine for polyurethane as it needs something to hold on to but for shellac it's way courser than I would be using. I would sand up to at least 800 grit for a good base. Putting shellac onto a 320 grit sanded surface on Vic ash will blow the grain up when the shellac is applied making the torn grain stand up and make the surface feel like someone has thrown a hand-full of sand into the finish.

This could also give he appearance of bubbles .

Third problem; sanding after first coat with 400 grit. Way too course and probably may have compounded the problem with raised grain.

• Would have been prudent to fill the grain first
• Sand no courser than 120 grit or even 180 grit.
• Sand through grits 180 240, 320, 400, 600, 800 and if you want the best finish 1200.
• Ideally sand with a random orbital sander or cork sanding block in preference to an orbital sander
• 1st coat 1 part shellac to 10 parts metho wipe over entire surface once.
• Leave to dry for a few hours or overnight.
• Wipe over the surface with 1200 grit to remove raised grain form sanding.
• If you're going to use a rubber use worm cotton or linen sheet as the cover.
• If you're going to use a brush go to an art supply shop or online and buy an artists camel hair water colour mop
• If you're going to do more shellac work with brush Gramercy Tools brushes also a brilliant video of brush being made I purchased a set of these brilliant French Ox Hair brushed some 15 or so years ago and they are amazing, although much dearer today than then.
• 2nd and 3rd coats 3-4 parts metho to 1 part Hard Shellac
• 4th coat 50/50 mix of metho and Hard shellac
• Final brushed coat full strength Hard shellac but unless you've mastered the brush strokes carry on with anothe one or two coats of the 50/50

If using a camel hair brush. Practise on the underside of the timber (you should coat both sides to stop warping and and sealing moisture and air out of the top. Also practise with rubber, they both take a little getting used to to get the best finish.

If applied correctly Hard Shellac shouldn't need to be sanded between coats as it adheres to itself unlike poly that needs to be roughed up between coats to give it something to stick to.

Hard Shellac and shellac finishes in general are brilliant and if you know what you're doing are easy to apply and get brilliant results. To a new user Shellac in general is a bit daunting and reasonably easy to muck-up big time.

If you really want to continue using shellac I would suggest getting a copy of "A Polishers Handbook" (bit of blatant advertising)

Think I've covered just about everything I can here. :yawn: Going to bed now. :sleep6:

Hope this is of some help and hasn't turned you off using the Hard shellac.

Cheers - Neil :U

PS For what it's worth.... The 1 1/4 inch camel hair mop will hold way more shellac than a 5" brush off the shelf no matter how much you spend on it. The mop should also give you a finish without brush marks.

Thanks all, I will sand back and try again. Following Neils instructions.
Alex