TOOLS

There are a number of high quality brushes that can be used for the application of clear finishes. The best for varnish and lacquers are natural hair (badger) or china bristle. Foam brushes are also an alternative to natural hair but wear quickly with usage. Use a new foam brush with each new coat applied. Rollers should be classified as non-shedding. Materials may also be sprayed by conventional, HVLP or airless application. 
Extreme care must be taken to prevent build-up and sheeting of the materials on the surface.

FINISHING

Special sealers are not needed to seal wood doors end will not do any better job than sealing with the finish product itself. Sealers can make the first cost easier to sand, thus speeding up production time.
Apply thinned material as sealer and allow drying overnight. Sand with 320 or 400 grit sandpaper. Remove dust with vacuum or tack cloth. Very porous woods such as Alder may require an initial sealing coat.
Apply the first finish coat to the surface by brushing either with or against the grain. The purpose is to apply material to the surface as quickly and with as little brushing as possible. Once on the surface, take one light pass with the tip of the brush (tipping) moving with the grain. Overlap each pass slightly, and then leave the material alone. Extensive back brushing can make the solvent evaporate quicker, reduce flow time and result in foaming or brush marks.

Allow finish coats to dry overnight and sand lightly with 400 grit sandpaper. Remove all dust between coats. When sanding, material should turn to a white powder. If the paper clogs, then the surface has not dried enough.

MAINTENANCE

Clear finishes do not require much maintenance. Surfaces can be cleaned periodically with a light soap and water solution as needed. DO NOT SOAK! Hand dry after each cleaning. A coat of paste wax or liquid polish may also be applied to renew gloss. Check with the manufacturer recommendation for usage and tips.
Interior clear finishes generally last for 5 to 7 years or more before new finishing is needed. Exterior doors, on the other hand, should be refinished every 12 to 16 months, as needed, especially in the extreme environmental conditions of the Desert Southwest. This may entail a quick recoat with the current product after a light cleaning and sanding, to an entire sanding of the surface with sealing and multiple finish coats.

Remember to follow both the finish product manufacturer and the door manufacturer's specifications and recommendations when applying finishes and hanging doors.