Sanding Floors - How to remove Tar-like coating first?

[subzero]

Sanding some old floors using a belt sander, for the most part this is doing a good job.
However, some areas of the floor are covered in a dark-coloured coating which just clogs the sanding belt up in no time. I've tried nitromors but this doesn't do anything. A heat gun and scraper helps but doesn't get rid of it all; what does come up forms a mound of 'tar' on the scraper. What could this coating be and how can I get rid of it? Presumably a drum sander would have the same problems trying to sand it.

I've attached some pics...

 

[Deleted member 174758]

Possibly the natural resin from the timber itself combined with dirt. Do the boards have a distinctive turpentine or pine air fresh smell about them? Either way I'd scrape the affected areas in the direction of the grain thoroughly then clean off with a Webrax (white semi-abrasive) pad soaed in white spirits. As ytou are probably now aware, any attempt to sand resinous timber simply results in the sanding material loading upand glazing or clogging in double quick time

 

[subzero]

Thanks, tried this but white spirit making no difference. Tried nail varnish remover and this took a little off but not much. It definitely appears that half the room has been actually treated with whatever it is, rather than resin as it's very distinct and the other half of the room is not affected.

 

[Deleted member 174758]

In that case it may well be down to hand scraping the floor with a Bahco 665 carbide floor scraper to get the stuff off.

Whilst these aren't as good as the old Skarsten-type heavy duty scrapers (which disappeared about 2001/02) they allow quick hand scraping of largish areas although it is hard work

 

[Doggit]

Can you turn the board over. The trick is to use a nail punch (or anything the size of the nail) to punch the nails through, and then lift the board out without any damage.

 

[Deleted member 174758]

That sort of depends - in our house the boards are T&G but only planed on the top surface. They also cracked rather badly when I tried getting them out. Hope the OP has better luck tha I did

 

[Doggit]

T&G always does, but these look like standard floorboards.

 

[chappers]

sounds like the blacking they used to use around the edges. What grade abrasive are you using. Anything finer than around 40grit will clog and then just reset spread around the adjacent boards.
I would start off with 40 grit or even 24 if it's really stubborn