Many well meaning DIY’ers will stain an exterior entry door, following good surface preparation practices, then apply a interior stain and varnish on the exterior side of the door. Using an interior finishing system will result in fading and peeling at some point down the road.
Interior stains use dyes and pigments that will not hold up to harsh sunlight. They will fade.
Interior clear coats will not block out UV light, and will break down when exposed to exterior sunlight day in and day out.
Minwax Helmsman urethane is an exterior urethane that comes in either water based or oil based formulas. It contains UV absorbers for ultraviolet light protection, and has the ability to expand and contract with exterior temperature changes.
Exterior stain
Start your exterior woodwork out right by using an exterior stain. Exterior stains have more pigment than interior stains which helps to block out harmful UV sunlight. They use exterior pigments too, which are more fade resistant. The tradeoff with these stains is that they are a little bit opaque in order to obtain their exterior durability.
Multiple coats
Many spar urethanes require two or even three coats for the desired protective effect.
Maintain the spar varnish
Since exterior films erode away (which is preferred to peeling), the spar varnish can wear thin after a few years of exposure, particularly on the south and west sides of the house where the sun’s rays are strong. Usually, you have to re-varnish every few years to maintain the film build and UV protection. But, don’t overdo the vanish application on the exterior either. If you build the film too much (by applying coats when not necessary), you can run into flexibility problems where the thick film build won’t flex any longer with temperature changes, causing cracking.
Recoating with spar varnish
Clean the door or exterior wood first, and follow that up with a light (but thorough) sanding. Use 220 grit sandpaper and sand in the direction of the wood grain. Be careful not to sand through the finish or you will end up removing the stain. Watch the corners and edges in particular, they are usually where the sandpaper will rub through the finish and start to remove the stain beneath.
Remove the sanding dust with a micro fiber tack rag, and recoat with a spar varnish.
Don’t sand the door or exterior wood surface if it is pre-1978 (the year lead paint was banned) Lead was used in some old (per-1978) stains and clear coats. Instead clean the door well and use a spar varnish (not a spar urethane). Test the adhesion first on a spot somewhere on the door prior to re-application (see cross cut adhesion test), since you are not sanding first, you may not get adequate intercoat adhesion, so check first before doing the whole door. Vanishes as a rule will adhere better (without sanding) than a urethane, but it is possible that even a varnish will not adhere properly without sanding, so check it first. If the varnish fails the adhesion test, either replace the old door, or bring it in and have it professionally stripped and refinished.
Related Links: Matching Interior Stain
Exterior varnish
Once again, interior varnishes will break down and peel if used on exteriors. The correct varnish (or polyurethane) to use outside is called a “spar varnish” or a “marine varnish”. You can get marine or spar versions of varnish or polyurethane, if you prefer urethane. Spar varnishes have UV inhibitors that will protect the varnish from sunlight which can enter into the clear film and break it down.
Please read lead paint warning if your house was built before 1978
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