Bitstreams wrote:

The beams in my cottage have been "painted" with a dark brown
substance that looks like something from those stop smoking adverts
twenty years ago. It's not quite gloss paint (though it is glossy) -
where the Neanderthal who applied it has gone over white lighting
flex, it looks a bit like thickly spread marmite.
I'd like to remove it and get the beams looking a little healthier.

What can I try ?

I'd ignore reccomendations for sand blasting or at least leave it as an
absolute last resort. I've just finished cleaning the chestnut beams in
my farmhouse and it has taken "quite a bit of time" to do a good job.
However having seen the mess that sandblasting made of both the wood and
the fabric of the house of a friend I was determined not to sand blast
these beams.

I cleaned them by hand using wire brushes. I could do about one room a
day which is as good as sandblasting can achieve. In fact better if one
factors in the clean-up costs. You will be eating abrasive grit for
years to come if you sand blast the beams, the grit gets in everywhere
and it's impossible to vacuum it all away.

I suggest that you test a small area to see how well it takes to a wire
brush. We also found scotch abrasive pads (not the pan cleaners, large
pads impregnated with abrasive) and wirewool to be useful. However some
finishes have a consistency like tar and wire brushes just smear it
around. For these Nitromors is possible or even alkaline strippers. Your
eyes are at particular risk if you try to use these methods on beams, so
you will need full-face protection when using them.

Sand-blasting is IMO the refuge of an oaf and it will result in damage
to the beams no matter what anyone tells you.