How do I test a varnish is oil or water based

[Oversanding]

I am refurbing some interior western red cedar joinery where the exsting satin finish has worn off all of the sun facing surfaces, but intact in others. I have no idea what varnish was originally used. Ideally I don't want to have to sand the whole lot back to the wood, and am hoping to just overcoat the existing surfaces.

If this is good idea (?) how do I test whether I need to use an oil vs water based coating?

Cheers

[Hobbyhorse]

If on holding a rag dipped in meths on to a painted surface the paint becomes a little soft, then it is an acrylic paint. Presumably the same would aply to varnish although I have not tried it on varnish myself.

Rhys

[munruben]

Yep , if you rub the current finish with a rag soaked in meths, if the existing varnish is oil based, then nothing should happen other than it will clean the finish. If the "varnish" rubs off or softens as mentioned above, then it is water based or acrylic type varnish.
Removing the old finish is, of course, the best way to achieve the best results but you should get a reasonable finish if you sand back the existing surface nice and smooth.
I would use a mat or low sheen finish so as not to highlight the underneath surface if it is not perfect. Unfortunately, the finish you achieve is dictated by the preparation you do prior to applying the finishing coats.

[Oversanding]

Thanks lads. Based on my experiences to date with this restoration, more sandpaper will be required!:C