I removed multiple coats of paint. Came off fairly easily but the external door has a slightly 'greasy' feel too it despite scraping carefully. The surface is smooth and looks ok but when I try sanding a few areas just to get it looking right for paint the paper clogs quite quickly. I am wonderiing if there is someting like varnish remaining and if his would affect my choice of paint? The surface looks a bit browner than I would expect.

What size and shape are these areas? If they are fairly small they are probably knots and require cleaning with meths and a couple of coats of shellac knotting on the top.

If it is all over the door it could be an original coat of varnish. If so do as Mr Sdoow says and wipe with some white spirit. If it can be sanded nice and smooth leave it on there and paint over it.