Most of the suggestions you'll hear are going to sound like the Indian parable of the six blind men describing an elephant from different perspectives.
Why? because there were/are a number of types of adhesives used to secure tubular tires to the rims. They require different procedures to remove the various types.
Your tires were glued onto the rims with the red Clement Gutta Extra rim cement. Over time it hardened up and in that condition the bulk of it can be easily removed. You can scrape it off with a plastic spoon, the round end of a butter knife or any other device that wont damage the rim surface. It should flake off.
If it's still a little soft you may have to use a chemical product to get it off (see warning below about paint removers).
Once you get most of it off the balance can be cleaned up with acetone, isopropyl alcohol, various kinds of paint removers, a wire brush, Scotch Brite, steel wool etc.
We called Clement red "gutta-rude-a" or "rude-a-gutta" because it was messy to apply and got soft when heated up from braking on a long fast descent. It would squish out onto the rim braking surface and squeal like a banshee.
It was also the holy grail among the raceur set because it was made in Italy and thus blessed by the pope! In reality, many of the pro teams used Clement red in the 70's... But what most "experts" didn't know was that the team mechanics mixed Clement red with Clement white plus they re-glued the tires or mounted new ones after every stage!!! They also had spare wheels carried by the support cars.
If you plan on reusing those tires, try to scrape as much of the gutta-rude-a off as you can so that you get a good fit when you remount them.
The next type was a clear, gray or white adhesive that remained tacky for a long time. This allowed you to mount a spare and continue riding if you got a flat. There were a number of different producers but Tubasti and Pastali were two of the more popular French brands sold in the US in the 70's. Clement made a white rim cement that sort of remained tacky but eventually hardened up.
Getting back to the parable of the elephant, using a wire brush on this type of cement will usually makes a gooey mess! Using a chemical remover is probably the easiest way to get this stuff off!
I used Pastali rim cement until I switched to high performance clinchers in the late 80's. Several years ago I dug out my old CX wheels. The last I time I glued them was in 1979. The Pastali was still tacky enough that I would feel comfortable riding them in a crunch to get back home.
A wire brush might that Pastali off after half the rim is worn away!!!
The next type of rim cement that some people used was 3M weather strip adhesive used to attach rubber to the doors and trunk lids (boot in the UK). It was yellow colored and dried quickly. The bond was very strong but there was no residual tackiness so unless you were riding track or had a support car you were SOL if you got a flat. Again, a chemical remover is the easiest solution to remove the stuff.
Shellac was used for mount track tires. Denatured alcohol removes the stuff.
Now the safety warnings:
If you use a rotating wire brush make sure to
wear safety glasses. A wire can break off and blind you!
Most types of paint removers/strippers contain DANGEROUS chemicals. Use them out doors and invest $25-$40 in a GOOD QUALITY respirator mask with filter cartridges that are designed for use with those types of chemicals. Your brain, lungs, liver and kidneys will thank you!
Now, the safer method of removing rim cement. 3M makes a product called Safest Stripped. It works pretty well and has no toxic fumes. It's available at many paint and hardware stores in the US.
Make sure not to get any paint remover on and brand name stickers on the rims.
Before remounting old tubulars, make sure that the base tape is well attached to the tire. The adhesive use to attach the base tape to tires made during the 70's and early 80's required a razor blade or X-Acto knife blade to separate the base tape from the tire casing:
Olde style base tape
Base tape used from the late 80's on was/is held on with a very poor quality adhesive and starts coming loose (or just plain falling off) as the tires age which makes for a scary situation.
What kind of rim cement do I recommend? None!
When I started riding sewups again about 6-7 years ago, I tried all of the different brands of rim cement that I could find. NONE of them were satisfactory compared to the old Pastali.
Then I tried Tufo Extreme Rim Tape. I'll never go back to messy glues again!!! It's fast, clean and holds well. I carry a spare roll in case I ever have to change a sewup on the road (see note below).
Note below...
When i switched to Tufo Rim Tape, I also started using Tufo Tire Sealant. There are other brands on the market but Tufo has always worked for me.
It wont work on most sidewall cuts or clinchers but it can seal up to a 3mm cut in the tread! I got several glass cuts in this tire. I put in some Tufo but I had to stop about every 1/2 mile and re-pump my tire. It got me back to the car!
The next morning I pumped up the tire and it held air - no problem!
I wrote this treatise in 2007:
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verktyg
Chas.