Bubbles in my spar finish...any ideas?

[driftwood]

Finishing some wooden dining tables for my galley. Just put on second coat of spar varnish. Keep getting bubbles or particles of some sort in it. Sanded between coats and removed all dust. Using new brush. What is causing these bumps? Any suggestions?

I am new at finishing wood....

Thanks

[Maureenm]

Not sure if it will help, but my aunt and uncle were furniture restorers. My aunt always had her varnish sitting in warm water, sorta double boiler style, and she always had beautiful results.

[Juneaudave]

Well...varnish is really susceptable to dust nibs and most of us don't have the ideal room to varnish in. Here is what I do whenever I varnish (at least on thosee final coats).

I try to clean up my garage really good the night before I am going to varnish....dust, sweep, vacume...anything to get rid of the sanding dust and dust bunnies. Then I stay out overnight so as not to re-entrain dust by moving around.

The day I'm varnishing, I take a misting bottle...like you spray use to spray plants...and lightly wet down the garage floor and work surfaces. That will keep the dust down, and I think it helps with the static electricity that seems to attract the dust to whatever you are working on.

Let me know if that helps...

[driftwood]

Dave, so you think dust is the problem. Must not take much to leave a bump. I was not even considering dust in the room, just dust on the wood from from sanding. Think I will try to be more careful on next coat...make sure it is really clean. Used tacky cloth to remove dust.

The thought of having to clean the garage makes me sad....

[Senior Ninja]

Varnish is to be stirred not shaken.
That usually takes care of the bubbles.
Perhaps a foam brush?
Steve

[driftwood]

Yep, stirred it instead of shaking. Will look into the foam brush and a total dust free environment and do a test.

[jonw]

Assuming you're using Spar Urethane varnish (Minwax), here's what I've found works best:

- Stir, don't shake
- Thin with mineral spirits 1:1 or 1:2 (mineral spirits:varnish) and apply more thinner coats instead of fewer thick ones - this will make the biggest difference in getting a good finish
- Use a brush for the first coat
- After the first coat always lightly sand between coats and use a tack cloth to remove dust just before applying the next coat
- After the first coat you can use a brush, foam brush, or even a small roller (since it's been thinned)
- With a roller, roll slowly to avoid bubbles in the finish
- To avoid thicker coats where areas overlap, brush/roll from an untreated area just up to the edge of the wet area you just finished
- For the same reason as above, don't apply in the sun if it is warm or hot outside

[Daniel Bernoulli]

Varnish gives off gas as it dries. Do not put a second coat over a first coat that has not fully dried. If you do, the second coat will trap gas and create bubbles between the coats. Let at least a day pass between coats.

[Galen]

I know with epoxy that if the temperature outside is decreasing instead of increasing in relation to the wood, you will get bubbles.

[driftwood]

Thanks, all good suggestions. I did not know I should thin it with spirits. It seemed to go on thick, and stil seemed a bit sticky several days later. How much sanding should I do? Just knock the shine off?

[jonw]

Thanks, all good suggestions. I did not know I should thin it with spirits. It seemed to go on thick, and stil seemed a bit sticky several days later. How much sanding should I do? Just knock the shine off?

Yeah, just enough to rough up the surface a little and remove any dust and crud that landed on it while it was drying.

BTW one of the advantages of thinning and applying a thinner coat is that it dries faster...

[Wolffarmer]

I know when I put motors together I like to wait until the weather has turned to calm and rainy. Which is about 2 days out of the year it seems here in Idaho. I sure don't put them together when it is dry and windy. That is just asking for trouble.

Randy

[Larry C]

Having varnished several kayaks, I have developed some good practices.
The thing I do is put a fan in a window of my basement, facing outside. I then take a leaf blower and blow the dust off everything. It all gets sucked outside quickly. Let it settle overnight. I then mop the floor, use a tack rag to remove the dust on the project. Lastly, and most important with any varnish, I strain the amount I will be using through a fine paint strainer. This will keep the tiny lumps that you can't see, off your project. The paint strainer is a must for me.

Larry C

[48Rob]

Thanks, all good suggestions. I did not know I should thin it with spirits. It seemed to go on thick, and still seemed a bit sticky several days later. How much sanding should I do? Just knock the shine off?

The varnish must be completely dry before you sand.
This time of year, depending on weather conditions and temperatures in a home workshop/garage, etc, it can take 4 days+ to be dry.

If you sand it too soon, it will gum up the paper and leave a bumpy mess.
If you are "just knocking the shine off" you are probably not sanding enough to remove any imperfections in the surface, which show themselves as "bumps".
The surface should feel very smooth, with the previous finish showing no "shine".
As with any varnish or paint, the finished product will only be as smooth as the prepared surface.
As in if the wood is rough and bumpy when the first coat of varnish is applied, it will likely be present in the final coat.


I must disagree with the poster that says it should be thinned to "make the biggest difference in getting the best finish".
It may work great for them, and for spraying, but thinning each coat is not recommended.

Thinning the first coat, or two, can be advantageous for getting better penetration into the wood.
Applied full strength, the final coats will/should turn out smooth and flat like a sheet of glass, with a great deal of depth.

Spar varnish is very thick compared to polyurethane.
It can be very difficult to apply smoothly.
Temperature and weather conditions (humidity) can make it hard, or easy to apply.
Pre warming the wood, and varnish can be very helpful if you must work in a cool area.

As has been mentioned, stirring, not shaking is extremely important.
However, if you get carried away with stirring, you can still introduce a lot of bubbles into varnish.
If unsure, just look into the can after stirring, if there are a lot of bubbles, let it set for a while and try to be much more gentle on the second round.

Rob

[Larry C]

The varnish must be completely dry before you sand.
This time of year, depending on weather conditions and temperatures in a home workshop/garage, etc, it can take 4 days+ to be dry.





Spar varnish is very thick compared to polyurethane.
It can be very difficult to apply smoothly.
Temperature and weather conditions (humidity) can make it hard, or easy to apply.
Pre warming the wood, and varnish can be very helpful if you must work in a cool area.


Rob

Rob..

I have to disagree with waiting 4 days to be sure the varnish is dry. From all I have read and in my personal experience, 24 hours is all that is needed. You don't want the varnish to be completely hard so successive coats will bond better. As far as sanding if you use the right dry sand paper or wet sand with silicone carbide paper, the varnish won't lump up on the paper. If your doing 6-12 coats of spar varnish, waiting more than a day is not going to work.

Quality marine spar varnishes have additives available to help them flow and keep a wet edge longer for different humidity. I use 2 different ones depending on conditions. These are not thinners, and only a tiny amount is needed to dramatically change the brushing characteristics.

I don't get too fussy on first several coats, just sand off the dust lumps. I usually don't have many because I strained my varnish first, and only airborne dust gets on the varnish. As long as the surface is sanded dull all over, it's ready for another coat. Sanding of the coat before the final coat is very carefully done, as anything that shows through that coat is there forever!

That's what has worked for me, your mileage may vary..
Larry C