I'm repairing a poorly designed area behind the kitchen sink which pools with water. I've coated the existing timber with a waterproof membrane, and then plywood over the top, as the existing benchtop is pine. I've used timbermate to fill the area, and was thinking I would seal the whole thing with a mixture of linseed oil and varnish. Will this be adequate?
Can timbermate be used in a wet area?
From what I'm reading this remains permanently soluble in water so there are better things you might have used to fill the area! But if you make sure that no water can get to it you should be all right, which is why you want to be thorough in sealing off the area from water ingress.
was thinking I would seal the whole thing with a mixture of linseed oil and varnish.
Don't bother with the linseed oil. If you want to waterproof the surface just use the varnish. Providing a water-resistant or waterproof surface coating is one of varnish's jobs, the level of protection based to some extent on the varnish type but mainly on the thickness of the coating. Adding oil to the varnish won't improve on this, just slow the drying time.
In this case any interior oil-based varnish of reasonable quality built up into a proper surface coat should be sufficient*.
Three or more coats of full-strength varnish is usually sufficient for a good surface coating, but it won't do any harm to apply one or two more if you want to err on the side of caution. If you're thinning the varnish for application you need to increase the number of coats accordingly.
Further reading from previous Q&As:
Protecting against water damage
*But note the water will find its way below the surface given the chance, so the edge of the sink insert and the rear edge of the counter must be protected with a bead of sealer such as silicone caulk.
You need to make sure you use marine ply when in wet areas. Covering up the old counter top with ply will cause issued in the future. I would remove the pine counter top, sand it, seal it and re-install it. When you use wood in the kitchen it is important to maintain it.
TimberMate is waterbased so isn't going to be much of a sealer. To be honest I stopped using TimberMate a long time ago. You get better results with sanding dust and glue, in this case you would use a water proof glue.
Just covering the old counter top with ply will result in the old counter rotting and causing all sorts of issues for you later.