The quality of the finished job depends not only on how thoroughly you prepare the surface, but the weather conditions at the time. Too hot, & the varnish can set before it levels, & bubbles left under the varnish can expand to distort the surface. Too cold & the varnish may not dry properly or will slump while drying. The temperatures that matter are on the surface you are working on, & these can be much hotter (or colder) than the local air temperature. If the work is in the sun in the morning, then paint that side during the cool months or favour the shade if the weather is warmer, then when the sun moves (or the tide changes) tackle the other side. The relative humidity can also affect drying time & quality of the finish. Generally above 85% relative humidity will produce less than perfect results, & applying varnish (or paint) when the air is close to its dew point is also undesirable.
Polyurethane products are best used when the relative humidity is less than 70% as the curing agents can react with moisture. Keep in mind that the conditions we are discussing here are those at the site. If you have sprayed a lot of water around to settle dust, then you will proabably have increased the local relative humidity. Some suppliers recommend thinning the first coat for better surface penetration, others recommend no thinning. Use only the recommended thinner & observe re-coating times. All suppliers recommend several coats for best results. If treating wood left in the open, more coats will give better protection, expect to apply up to 8 coats. When brushing, use firm diagonal strockes at first, finishing with a lighter stroke along the grain. If using a roller on large flat surfaces, charge the roller & then start applying the varnishing in the shave of a W, finishing with a run along the grain. Use a fine broad brush to tip out any bubbles to product a fine & even coat. Most suppliers recommend this be done with two people, one on the roller & one on the brush although it is possible for one – work as fast as you can. Once you begin, keep going; do not allow the wed edge to dry as this will leave a visible line. Try not to brush the varnish too much; the liquid surface will self-level, brush marks disappear.