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I Have an outside deck, it currently has a layer of varnish on it, I Think the wood type is Meranti.

The deck is exposed to lots of rain and also harsh sun, and the varnish has started to peel.

I Would like to re-seal or repair the finish

What would be the best type of product to use, (varnish, solvent based sealer, oil based sealer, water based sealer) etc... And if so, how would that affect the current flaky varnish?

I would essentially like a semi-maintenance free application where the wood luster is maintained.

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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.

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There are two different sets of recommendations depending on the type of decking:

For a pressure treated lumber deck:

To start with, you will want to remove the current flaking varnish, otherwise any treatment you give the wood will not apply consistently, and in any case will just sit on top of the varnish, so strip it all with a good paint stripper. An oxygen based one can brighten up the wood quite considerably as it gets rid of mildew.

Wash the wood - a power washer is pretty good for this.

You may also need to sand - this helps take the surface to a consistent level, and can remove splinters that may have been left after the paint stripping and washing.

Then use an oil based stain if you want to go for long term wood protection - this will get right into the pores in the wood. Ensure the deck is dry first - a couple of days of sunshine and no rain will help the uptake of stain. Make sure the stain is semi-transparent if you want the lustre of the wood to show through.

Varnish and paints should not be used - You have seen what UV rays do to varnish.

For a hardwood deck:

You will not want to use such active chemicals to strip the paint, and in fact you may wish to just use a sander to avoid chemical damage to the wood.

An oil based stain will not be as useful either, so you may want to go with a more resin based stain, or even just re-varnishing it.

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This is solid advice for a typical pressure treated lumber deck. It appears this particular question is referring to a hardwood deck, which would likely require altering these instructions (e.g. non-oxygen based stripper, stain choices, etc.) –  James Van Huis Nov 8 '12 at 23:15
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