I've used in the past foam brushes to apply my water-based varnish. I'm wondering if there's other types of brush that can be used and what are the advantages and disadvantages.
Generally, besides foam there are synthetic bristle brushes and natural bristle brushes (as well as sprayers, air-brushes and rollers, which I'm not going to discuss). There is great debate over which to use for what; that debate rages from house painters to fine artists.
Most agree synthetic bristle brushes for water-base varnishes, acrylics and the like and natural bristles for oil-based mediums (the solvents can damage some synthetic bristles). For me, both types work well but have slight differences in how much paint is held, how the paint leaves the brush, etc. I find I 'get use to' how the brush handles fairly quickly.
There is the question of shape, which for most furniture folk, is less of an issue unless you are doing decorative painting, stenciling or similar work. For transparent finishes, a flat tipped brush is most common. But if you have something else that you like, great; use that. Your experience with foam will guide you regarding shapes you like (for getting into corners, for example) and widths.
While slightly off question, it is worth noting that you can tweak your finish to work with your brushes. For example, by adding more thinner (water in your case) you can create a thinner coat that dries with less visible brush strokes. This also applies to binders and in the case of paints, the pigments as well.
Would it be proper to note that by adding more thinner to whatever you are using, you'll also run the risk of runs/drips ... not that this will absolutely happen, but you'd need to take more precaution? My front door is mahogany. I put marine spar grade varnish on it once every two years after thorough cleaning and steel wool rubdown. If I'm not careful on the vertical surface, runs will show very easily ... thinned down, it would occur even more quickly. Just my case-in-point. Apr 3, 2015 at 15:18
Very true, and we haven't touched on technique or setup. I only mentioned it as an example of how tweaking the finish changes the equation!– ewmApr 3, 2015 at 17:30
I'd recommend spraying water-based varnishes. They dry out really fast and, unless you're working in a very small area, it's easy to get brush strokes showing in the finish.
I never really thought of spraying varnish. Is there a lot of waste when spraying? How can you correct or avoid over-spray? Apr 3, 2015 at 15:51
The two methods I use to manage overspray are masking and a paint booth or area to block wind and that you don't mind getting painted. Allowing some overspray gives a better finish than fighting too hard to control it. (start your stroke, trigger before you get to your work, release after you cross your work, then stop your stroke.)– hildredApr 3, 2015 at 17:23
Overspraying is going to be an issue, even if you have the pressure just right. It's one of the disadvantages of spraying. But, you avoid brush strokes.– dfifeApr 7, 2015 at 16:53
Synthetic brushes were originally specifically intended for application of water-borne finishes. Compared to natural hair brushes synthetics are better in one key respect: they don't soften in water. Natural bristles "lose their spring" when wet, they are still usable with waterbase finishes but they become slack and unresponsive so they don't perform at their best.
The best modern synthetics have a mixture of filaments of varied diameter and they may in addition taper towards the tip, both of which contribute to improved capillary action (helping to match the performance of natural brushes). Another feature of better modern synthetic brushes is ends that have been frayed, which mimics the natural 'flags' found on traditional hog-hair brushes.
In my experience foam brushes are not ideally suited to application of waterbase finishes, primarily because they increase the tendency of bubbles to form as you brush over the surface. Because of this I think foam brushes are best for oil-based or spirit-borne finishes which are less prone to bubble formation.
If you are doing a large area you might consider a popular method used to apply water-based polyurethane to wood floors - which is to use a "T-bar" applicator that has a thin foam or fleece-like cover.
The applicator is not intended to absorb much material and again release it the way a brush does. Instead it quickly reaches saturation and then just pushes the liquid around on a flat surface. This allows for a very smooth and quick application. Apparently the finish also comes out very even I guess due to its own viscosity and the minimal weight applied to the tool; you apply no down-pressure at all.
I doubt this could be applied to a small project but if you were doing say a tabletop it might work very well.
As a caution, it could be that a method intended for a floor would leave some imperfections you wouldn't notice on a floor, but that nonetheless could be visible on something closer to the eye. But the floor I did with this method didn't appear to have any flaws related to the finish that I could see even when I looked closely.
I use golden taklon brushes for WB finishes. The very fine bristles dramatically reduce brush marks. Also, the quality WB finish manufactures also sell an extender/retarder to slow the drying, necessary if brushing something of any size with waterborne. It is better to spray them.