Background: I recently replaced my home heating system (an oil burning furnace) for a modern heat pump (air-water). There is a room in our basement (2.9m x 3.5m, or roughly 10ft x 13ft) where the 10'000 liter (~2500 gallons) tank used to be. The room was completely sealed off from the basement, save for a small window-sized opening through which you could enter to service the tank, should that be necessary.
I had a contractor cut out a door opening, and then a specialized firm came to clean the tank, cut it into smaller pieces, and recycle the steel chunks. I have now and empty room that reeks of oil, which I want to turn into a home gym. It's been some 4 weeks since we cut a door opening, and opened up the grilled vent to the outside, and although I've let enough air circulate, it still reeks as much as it did on the first day.
I assumed some oil may have leaked through the years onto the floor, which had a sort of epoxy coating, stained brown in some places. So I carefully removed it all. It took a lot of effort with a putty knife and paint scrapers. But even with a completely clean floor, it still smells. The lower half of the walls, which have the same paint/epoxy coating, have brown stains all over, which I imagine may have significant deposits of oil, perpetuating the lingering smell in the room. I read that spraying concentrated hydrogen peroxide helps eliminate all smells, so I carefully sprayed 12% hp on the walls (wearing a full body suit, goggles and a breating mask), which didn't have any significant effect overnight.
The question: is there any reasonable way (that doesn't involve overly expensive or extremely time-consuming efforts) to get rid of the smell, before I apply the decorative textured plaster on the walls? Would a controlled burn (e.g. with a small gas torch, with proper ventilation) help at all? Do you think applying plaster over the existing walls wouldn't do enough to cover the oil smell?
Edit / Update: on Friday I primed and then used self-levelling compound (about 750lb / 350kg) to raise and level the floor, which had over 1 ¼ / 30mm height difference. The smell is completely gone. So as several of you pointed out, the oil had somehow leaked and penetrated the floor, which is now completely covered by a non porous concrete layer. Thank you all for your kind participation !!!