Wood – How to seal seams between wooden deck rails

Paint is a sealer of sorts. If the paint is peeling, then the issue is the paint itself. Perhaps the wood wasn't prepared right, or there is no proper primer, or they didn't use paint specifically for foot-traffic (they make paint specific for floors/stairs that wears better with foot traffic).

Your best bet is to strip what's there and start over. Sand it down a bit, use a proper primer, etc.

What types of sealing products you need to use will depend on the type of final finish you want to see. If you have a varnish type finish now, that means you are seeing a clear, transparent finish. You didn't mention if the color of the deck has faded and needs to be refreshed. Refreshing the color would be a complete step to itself. Let's adress the peeling of the old finish. If you want the same end result as you had before the peeling occured, then take the following steps.

First: Remove all loose varnish and sand the entire surface with 150 grit paper. A DA sander is good for this. Assuming the wood color is OK, you do not need to remove all the old varnish, just sand it to remove the gloss and any dirt etc. Remove all the dust, clean thoroughly. Apply a couple of coats of a Spar type varnish. Spar varnish or spar urethane is typically used outdoors and in marine applications. Be sure to give the new finish a couple of days to cure before walking on it.

If you want to completely replace the varnish finish, then you will have to remove all the varnish to bare wood on the entire surface. At this point, you can decide on a new finish. There are lots of choices that would work in your application. If you want to see the natural grain of the wood, then select a transparent stain to re-color the wood without hiding any of the grain. Outdoors, I would prefer a linseed oil based stain formulated for decks, not siding. There is a difference. Getting full oil based stains is getting harder to find today as most have gone to a modified oil base with acrylics added. Again, you could use an oil based spar urethane overcoat to seal the color and waterproof the surface.

Other single step finishes (after removing all old varnish) would be transparent or semi-transparent deck stains. Check with your paint supplier to compare the options. Formulations of deck stains vary as do prices. Lower quality paraffin based products are cheaper and don't last as long. The high end of the spectrum would be something like Sikens oil based deck and dock stain at around $45.00 a gal.