Blisters with Varnish on Cedar Outdoor Cedar Tabletop

[kkahler]

I should start by stating that I have very little experience with wood finishing, but I have done my share of reading on the topic. I purchased a pre-finished natural cedar tabletop last year (finished with Hellsman spar urethane). About mid-summer, I started to notice bubbles appearing on the knots (and only on the knots). During the winter, I stripped the table down to the bare wood. I applied 2 coats of Bulls Eye Sealcoat (apparently can be used under spar varnish) to seal the knots, assuming this would prevent the knots from bleeding again. I followed this up with Epifans high gloss varnish. I applied several coats with graduated amounts of thinning according to the directions on the can, then finished with 2 coats of 100% varnish. The table was finished about 2-3 weeks before it made its debut outdoors.

On the first day with direct sun, the table was covered with blisters (not just on the knots). I have read this may be caused by the shellac, but I’m not sure? I’m quite frustrated with the result at this point, and before stripping the table again, I’d love to get some advice or suggestions on what might have gone wrong or what I should try next. I imagine it shouldn’t be that difficult to get a good finish on a piece of wood to use outdoors.

I’d appreciate any help on this. Thx.

[router]

In your commentary you do not seem to mention where the shellac was used but later on you seem to indicate that this may be the problem.
In my opinion the bubbles would be a reaction between two or more of the products that you have used.
As an example oil based products and lacquer are not compatible and will blister.
The blisters may also be caused by moisture and/or silicon on the surface.
Silicon tends to go in the grain and is nigh impossible to get rid of.
If the timber has been previously polished with something containing silicon then that is a problem.
Almost anything will adhere to shellac so in my opinion the blisters would not be caused by shellac.
If it were me I would sand it back to bare timber and most likely use a good sanding sealer and then use the appropriate top coats making sure that they are compatible with the particular sanding sealer and not mix different brands of products.
All the best.
Router:roll:

[kkahler]

I coated the entire tabletop with the shellac (two coats) before using the Epifanes varnish, but I understand that the Bulls Eye Sealcoat isn't a real shellac? You mention a sanding sealer, can you provide a recommendation on what product to use...I not familiar with what that is, and if it is different from the Bulls Eye Sealcoat that I used.

I have no reason to believe any silicon was on the tabletop at all. I bought it as an unfinished raw piece of wood, and the place I purchased it from finished it with the Hellsman's spar urethane.

Thx

[Xanthorrhoeas]

I am wondering who told you that the timber is cedar? There are lots of different "cedars" though the most commonly used outside timber described as cedar is Western Red Cedar (WRC), a North American timber that stands up to the weather well. Your table does not look at all like WRC, nor any other timber I know that is described as cedar. What it does look like is Australian Cypress Pine (A Callitris species) which is also very weather and termite resistant and has the distinctive two-tones of the light coloured sapwood and the darker heartwood.

If the timber is Cypress then it is a naturally resinous timber that may explain your finishing difficulty. I have successfully used an oil finish on Callitris outdoors but the oiled timber does develop crusty sections where the resin leaks out of the timber.

Hope this helps. I am sure there will be good advice out there once you are sure which timber you are using.