I have to remove remove some mastic from a concrete floor where there were vinyl tiles. Is a hot air gun enough or do I need my propane torch. Are solvents a good idea? Second problem: Mastic again. This time on a concrete wall that is probably going to be painted. What's the best way to prep for painting? Thanks Karl
Many tool rental places have a tool called a scrape-away tool, a circular scraping attachment which fits a floor buffer. It has steel blades that ride at an angle on the concrete and scrape the mastic off. You can get more done with one of these things in 45 minutes than hot air, sanders or solvents combined. I've used this gadget a number of times and it's always been a labor saver. It may be called a different name in your locale, but describe it to the rental houses and you should be able to find it.
The best point of beginning is one of the razor scrapers about half way down this page:
will work quite well on floors and walls. Heat, as in a heat gun or propane torch will soften the material but you will probably have better luck just scraping cold. The floor disk that was referenced earlier cost a little over $600, I don't know about rental. They do not work well in heavy cut back adhesive (black). Avoid flammable thinners,etc. If you want to try chemical, get some citrus solvent from a commercial floor installer's store. I still think your best bet is a good floor scraper with plenty of fresh blades and a little elbow grease.
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I know that if you get it cold, the mastic gets brittle like old tar chunks. That said, you can get some dry ice and put it where you want to scrape. Not sure how well it will work, but it might be worth trying.
I read somewhere that you put a box of dry ice over the vinyl tile you want to remove and after moving it out of the way you smack it with a hammer and tile leaves in lots of small pieces. Same idea.
If you've got a source of liquid nitrogen, that might be another answer (well, for the floor at least). Note that both dry ice, and liquid nitrogen, will displace oxygen from the room. If you enjoy breathing, caution is advised (lots of ventilation). CO2 will pool, which is inconvenient if you pass out. Liquid nitrogen is inexpensive _if_ you have a local source.