I've strained varnish through cheese cloth layers and the filter cones paint stores sell and really like the filter cones. If there is gunk in the can you need to get it out.
I'm not an expert, but here's what I do to get the best results I can. This assumes gloss varnish.
If you are working over a cement floor, vacuum it thoroughly the day before you varnish, and wet it down the day you varnish to keep dust down there. If the room has lots of dust you oughta vacuum everything once in awhile. Don't stir up dust when you get ready to varnish. If you are working in a garage with an electric door opener unplug it! (long, sad story there.) Don't use a fan.
Sand until smooth, whatever that entails, vacuum, and tack cloth the area to be varnished.
Buy a really high quality marine spar varnish with lots of tung oil. I hate poly varnish because it looks cheap and and is impossible to repair properly if it gets chipped. Poly forms a giant plastic coating as opposed to a unified film. Gently stir the varnish if it needs to be stirred. Never shake the can. Read the mfgrs. directions and follow them. Pour some varnish into a clean disposable plastic paint cup you buy at paint stores. Never varnish right from the can.
Buy a really good varnish brush and expect to pay way more than you want. Keep the brush wrapped up until you get ready to use it and then knock any loose hairs or dust out by knocking the bristles rapidly back and forth over the hand not holding the brush and gently remove any bristles that are loose. If you clean it after every use and care for it well it'll last for years.
Load the brush with varnish just half-way or so up the bristles, gently tap the brush on the side of the cup above the varnish once to get the drips off, and lay the varnish on with long, slow, smooth motions overlapping adjacent strips and working from a wet edge (just varnished) out over unvarnished areas. The varnish should smooth out and self level. If it doesn't or is not brushing on well, thin it, even though the can will say not to. The can is trying to remain VOC Compliant. not help the varnish flow. If the varnish shows brush marks even when thinned you can "tip-off" by holding the brush perpendicular to the work and only using the very tips of the bristles, smooth the varnish out, sorta like smoothing icing on a cake, but different. I always lay down the first coat at less than full-strength, but not as weak as 50\50, more like 60\40 - 70\30, depending on how thick the varnish is and what I'm covering. It's a feel you'll develop. If you are getting bubbles you need to stop, load the brush with varnish and gently wipe it over the edge of a coffee can (if you can find one!) to get the foam out. Keep your brush loaded with varnish and proceed slowly. Apply several thin coats and no heavy coats. Don't load the varnish up to the metal band holding the bristles.
Carefully walk away and forbid anyone to enter the varnish zone for 5 hours. If it's bug season turn the lights off so they aren't flying into the varnish.
After the initial coat has completely cured(sanding results in a white dust not gummy sandpaper) sand everything with 220 grit wet or dry sandpaper on a rubber sanding block for the high parts, wood or a dry celulose sponge for the low areas between ribs and such, folded a few times for the picky detail areas, wet with clean water, and wipe it off with a clean, damp, lint free cloth (old, clean diapers are da' bomb!), folding the cloth to keep a clean part outside as you wipe. Let this dry and vacuum and tack again.
*Note - If you don't use a sanding block the sandpaper won't knock off the high parts of the bubbles and specks, but will follow them and you won't get a smooth finish.*
Apply the next coat following the same rules, thin if you need to to make the varnish flow from the brush instead of 'grabbing' the surface of the work.
Varnish, rinse, and repeat until you get the depth you desire, I usually do one thinned coat and two full strength.
*Another NOTE - Let each coat of varnish completely cure before applying the next layer or the lower layers never cure properly, resulting in ugly and disappointing results.
After a couple of days you can polish the finish out with a variety of things from the 3M series of Finesse-it polishes to ultra fine steel wool to 800+ grit wet or dry sand paper, if you want.
When dings happen dry the area when you can, sand with 220 - 400 grit, tack and apply a couple of light coats of the same varnish to the dings, allowing it to dry between coats, until you have filled the boo-boo. Sand lightly to blend into the surrounding area.
If you just do the dust elimination, surface prep, get a good brush and varnish, watch what you are doing and the results you are getting, and adjust as required, rub the dry varnish down with 00000 steel wool and tack well between coats you'll get a good finish, just not showroom quality. Often this is good enough. If you do a great job on the decks, wales, and seats most folks will be plenty impressed providing you do a pretty good job on the rest.
Again, this is from my experience. Your mileage may vary.
