01-23-2010 07:20 AM
I would think that it would be your own preference. Preparation would be my concern. Although I like oil based varnish for my wood projects...you might like water-based for this application because it doesn't smell as bad as oil and the clean-up is easier.
01-23-2010 10:53 AM
Not from what I have experienced. You need to rough up the surface before you apply.
01-23-2010 10:24 PM
Now, IF I remember correctly, my dad used to varnish his doors this way: Scruff sand the whole door, and wipe everything clean. Door would be laying on a pair of sawhorses, using the hardware ( he left the hinges on, and the door knobs) to keep the door off the sawhorse itself. One coat, allow to dry until no longer tacky to a finger-tip, and then he grabbed a rag and hand "rubbed" the varnish until there was an even "shine" all over the door. He allowed that door to finish drying (about a day) and then rehung that door. Why did he leave the hardware on? To flip the door over and do both faces of that door. He grbbed the door knob and flipped the door over. Simple.
01-24-2010 04:02 PM
While there is a chance that whatever you apply to a cleaned surface, it will stick, there is also a chance it won't do so well. I think it would be worthwhile (considering the risk of effort to have it to re-do from scratch) to check the type of existing finish. Most finishes stick best to their like-kinds. There is a "What is that finish" test on http://www.hardwoodlumberandmore.com/tipsheets/wha
Knowing what might have been originally used (onsite brushed, sprayed, or factory finish) might give you some clues. Some finish painters/finishers are extremely talented; others have skills that end with latex wall paint and will apply Helmsman Spar Urethane to built in desks or use Polyshades to darken a pickled handrail (in other words, complete idiots).
If you have problems determining what it is, or conciously choose to switch from one class of finish to another, a barrier coat of dewaxed shellac would be good insurance.
That's my dog.
01-25-2010 02:43 AM
...is why you want to put on a new coat of varnish. Is the present finish failing in some way? Has it become dull, or possibly damaged? Have you tried anything to clean the existing finish? Do you know what finish is currently on the door (the link provided by KMealy can help you make that determination, but it won't distinguish between types of oil-based varnish)?
Can water-borne acrylic be applied over oil-based varnish, and/or can oil-based varnish be applied over water-borne acrylic? Sure; but the question is will the recoat be successful long-term (possibly even short-term). If, for example, you know that the existing finish is oil-based poly then there is a high likelihood that any new finish that contains "poly", whether oil-based or water-borne, will experience adhesion problems. Poly, in whatever form, does not adhere well to itself. The adhesion issue becomes even more of a problem if the existing finish is fully cured and/or if there are "contaminants" such as wood care products, household "gunk" (kitchen oils, etc.) on the surface. "Scuff sanding" between coats of newly applied poly on fresh, bare wood surfaces is a critical step in improving adhesion between coats of poly. However, the same sanding on old, cured surfaces is far less effective. The simple reality is that poly to poly adhesion is poor.
If the existing finish has begun to fail, or has been damaged and you don't know what finish was used then your safest course of action, as already suggested, would be to strip the door and begin again. If the finish is sound a good cleaning may be all that is required. Begin with mineral spirits/paint thinner and plenty of blue paper shop-towels. Wet the surface of a towel, wipe, turn to a clean face and wipe again. With every wipe expose a clean face on the towel. In this way you will remove the oil-soluble grime instead of just softening it and moving it around. Follow the mineral spirits/paint thinner with a good wash down with detergent and water. It is amazing how many times a good cleaning will successfully "rejuvenate" a dulled surface.
If after cleaning the sound finish you still want to apply another coat and you are determined not to strip the existing finish then apply a "barrier coat" of shellac (Zinsser SealCoat® which is marketed as a "sanding sealer" will work well). The shellac will dry very quickly so don't spend a lot of time fiddling with it; apply it quickly with a soft, natural bristle brush and move on. When the shellac is dry (30-minutes or so) lightly sand as necessary to smooth the surface with 320P open coat sandpaper, wipe the surface clean with a shop towel lightly dampened with ms/paint thinner and apply the finish of your choice. The shellac will seal in contaminants that might otherwise cause "fish eye" and will prevent adhesion problems, even if you use polyoneverythane.
But again, stripping with a chemical stripper and completely refinishing is to be preferred...