Avoiding Bubbles with Spar Varnish

Brush technique is key. April 6, 2007

I used Helmsman spar varnish on a few half-log benches because I read it was the best choice for wood that expands and contracts quite a bit with the changing seasons/humidity. It's also waterproof and the most durable finish you can put on a wood project. The problem I had with Helmsman spar varnish is that I would get tiny bubbles in the wet varnish as I applied it to the wood with a bristle brush. I bought the bristle brush at a local hardware store. Could the problem of tiny bubbles be with the brand of varnish I used, or could it have been the brush? Should I try a rub-on spar varnish? All of my projects are currently in limbo.

Forum Responses
(Finishing Forum)
From contributor M:
It might be you are overworking the varnish. Because varnish flows out very easily, you should not have to make double or triple passes with the brush. One of the tricks is knowing when it's time to reload the brush so you can make enough single passes to cover a certain distance before reloading. Open the can and stir it up, then pour some into a clean container, close the can, and allow the varnish to settle. Always check the thinning ratio. Never pour the leftover varnish back into the can. Do not over brush - this will create bubbles.

From contributor A:
One trick I heard about is to tap the excess varnish off of the brush. Don't screed it off on the lip like you would excess paint. This works air into the bristles which will flow out as you're applying the finish. I've done this with polyurethane, but I assume varnish would react similarly.

From contributor T:
Dust in the finish and bubbles are probably the most common problems with varnish.
1. Use a very good brush. Should be chisel edged natural bristle with tapered, flagged bristles.

2. Thin your varnish 5% - 10% with mineral spirits. This will allow the varnish to flow out better and slow the cure time so any bubbles have more time to pop out.
3. Press out excess varnish by pressing the loaded brush against the side of your container, not by scrapping it over the edge of the container.
4. Don't over brush. Bubbles result from the bristles mixing the varnish as you brush.
5. Tip it off. This is done by holding the unloaded brush almost vertically and gently making a pass completely over the finished surface.
6. It's a good idea not to dip into the original container and not to return unused varnish to the original container. Both practices can introduce dust into the varnish.

You can thin the varnish way down and wipe it on, but this usually causes it to cure harder, which is not what you want from a spar varnish.

From contributor M:
You might want to try using a poly foam brush with the varnish. I have had good results with both varnish and polyurethane and foam brushes. They come in several different sizes. Follow all the advice that has been posted here and you will get good results.

From contributor P:
Helmsman is not the best varnish you could be using. If you are looking for a product to expand with the wood, I would suggest a Sikkens product. Cetol 1 or their door and window products are both unreal. Also look into Corona brushes. I swear by their short hair marine brushes. Thin product out a bit! Another great product would be one made by Epifanes.

From contributor N:
Read your can of Helmsman. If I recall, it isn't even varnish anymore, but a urethane.

From contributor A:
Several of the high end sailboat builders I've visited in Maine use the disposable chip brushes. It's very hard to get a good brush clean. Helmsman is not your best choice by any measure. Epiphanes varnish is regarded by most as the best in the boating community. It's a little pricy.

From contributor T:
Chip brushes might work if the varnish is a good one, but the problem I have is the bristles keep coming out. If you dip your dry brush in MS before you start, you'll have a much easier time cleaning it when the time comes. I think it makes the varnish flow off better too. Helmsman wouldn't be my first choice either. If you really want varnish, I'd add Waterlox Marine to the list of alternatives. Sikkens is probably about the best product you can get for an exterior finish. As far as I know, oil modified resins are called "varnish". There're lots of oil alternatives and three primary resin alternatives: polyurethane being one of them. None are "waterproof".

From contributor O:
I would agree with those that chose Cetol. It goes on easy - no bubble problem, and as time goes by, you can recoat right over the original without sanding. Cetol does have a slight reddish color to it, for lack of a better word, at least what I have used. I don't think I have seen a clear. Now, if you want this finish to last for 4 years or more, as many as 10, put a clear over it called Bristol. It is amazing. Not cheap and can have a bubble problem also if not done correctly. As everyone has stated, do not over brush, use a decent brush, and do not shake the can, but stir lightly. Bristol is a marine product and is not in all marine stores. I also think Helmsman is not the best product.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor R:
It sounds like contributor T has good technique. I would add that using a good quality ox hair or white china bristle brush helps. Also take care to keep the brush saturated and be deliberate keeping the brush at a fairly consistent angle and pressure on the work. Feather out the end of each stroke and avoid any kind of flicking or separation of the bristles. Remember that it only take one bad move to introduce those bubbles, but it takes a lot more effort to get them out.