To protect a reclaimed table top, could I use gloss poly as the first coat and then top it off with a flat poly? I've heard that gloss gives better protection, but I want a flat finish.
could I use gloss poly as the first coat and then top it off with a flat poly?
Yes you can do this, in fact many good guides recommended that you do this. Where you want to build a substantial varnish finish (e.g. more than four coats) for a very good level of protection it's best to use gloss for the first few coats then use matt/flat varnish only at the end as a surface coating.
This prevents a few issues, but primarily you don't have to worry about successive coats of matt varnish combining to create a veiled or cloudy effect which hides too much of the wood's character.
I've heard that gloss gives better protection, but I want a flat finish.
Another thing to consider is that you can create a matt surface on the gloss varnish which will obviously save you having to buy two types of varnish.
It's really quite simple to do, all that's required is creating a fine and uniform scratch pattern on the varnish. In the past this was commonly done by rubbing down with fine steel wool (000 or 0000 grade) but today many people use the finer grades of Scotch-Brite. I'm more of a fan of steel wool because I think it's slightly easier to use for this purpose but both will do the job, and Scotch-Brite lasts much longer so it is a better buy.
When rubbing down varnish it should be allowed to dry well so that it is good and hard. How long this takes depends on how thickly it was applied and the local temperature and humidity, but as a rule it's safest to wait at least a couple of days after the last coat was applied, and waiting a week or longer wouldn't be a bad idea in colder weather.
In practice you know if your varnish has become hard enough if there is no gummy, dragging feeling as you go over the surface and the dust created is fine and white.