I removed a throttle body which had a brittle, baked on gasket, part of which adhered to the intake manifold and part of which adhered to the throttle body.
How do I get the baked on parts of the gasket to come off? They're hard as a rock - I tried scraping them off with a knife, but I'm a bit afraid to damage the mating surface, causing it to not seal properly afterwards.
From my experience, using a putty knife with a sharpened edge (single bevel) is a very good way to go. I'm taking from your question, you are using a regular knife (one with an overall curved edge) to take it off. This is not a good way to go because the curvature can definitely cause you issue. A putty knife with a flat, straight, sharpened edge works really well, along with the chemicals which @Zaid mentioned. The key to not damaging your surface is to ensure you keep the blade straight (flat/flush) to the mating surface. Also, put the beveled side down. This gives you more control over what the knife edge is doing. Find an edge in the gasket and work on it. There is no getting around the work part, as it is tedious. Take your time and work at it. You'll find what exactly works. I cannot describe exactly how to do it; only experience will explain it.
Sharpen a wood chisel and it will take it off pretty easy in hard to get at places, I just did it on the block changing my water pump, very nice, cuts in and scraps off : )
Burnt or baked-on rubber gaskets can be really hard to remove!
There are dedicated chemicals available for this job (unsurprisingly called "gasket remover"), which make scraping off the old gasket really easy.
If this isn't available, it is still possible to scrape it off, but will require lots of patience and finesse.
I've heard of transmission shops in India that utilize regular paint thinner to remove old friction disk material, so it might be worth a try.
In my own experience, brake cleaner is not well suited to the task; it will be wasted on it.
From what I've learned recently that there's this really great mechanical rotary attachment made by 3M that can really get off gasket material safely without damaging aluminum. It's specifically designed for automotive gasket removal one of which is dedicated for aluminum, the white disc
This fella here walks through the white, yellow, and green discs, which increase in roughness as you go up. Here's the relevant quote from his video, and here's where he's using the tool, the white disc on a cast iron engine block's head surface...
Now what I use is these 3M Roloc Bristle Discs, and they're the best option out there as far as removing gasket material quickly and safely. Now they do produce grit just like any other abrasive so you need to be careful with the grit and where it goes. But there is different grits as far as how aggressive they are. It goes from 50 grit green, to 80 grit yellow, to 120 grit white. Now what Ford recommends is to use the green on the actual cast iron surfaces and to use the white on aluminum surfaces.
Here's another use, this time of the yellow 3M Roloc bristle brush on an aluminum head. He refers to the disc as a metal safe cookie wheel. He also does the same treatment to the throttle body mount to the intake manifold just shortly after the above.
There's also chemical means like the CRC 05021 Technician Grade Gasket Remover which I've read you use a special plastic knife with after the chemical spray softens up the residual gasket material.
