Hi conks, if it's really thick then the chances are that it is paint. If you do intend burning off the old stuff then you will need to be very careful as over the years I have seen a few houses catch fire because of people doing this, even professionals. It can be done but you need to take a lot of care while doing it especially around the sash boxes. I was taught to do burning early in the morning and to be finished burning by around lunchtime so that you have a few hours afterwards to make sure everything is ok before going home. If you haven't done it before you may be better off using paint stripper. After you have stripped it off make any repairs, give a good sanding before putting on stain or paint. Paint will be thicker than stain and will take at least a couple more coats. If you do use paint make sure you leave the room for this between the windows or they will stick.
1. I wouldnt put a heat gun anywhere near a pain of glass, being sash windows they will more than likely be single glazed. therefore i would use nitro moers.
2. Have a close look at the sills if you can see the grain of the timber through the paint then it will be a stain varnish. If its smooth and solid then its a paint finish.
If you dont wish to strip give it a good sand and apply an undercoat to the window which co insides with the gloss/eggshell/satin finish which ever you desire.
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