If you are getting vibrations from weights being "off", then I'm betting it's not marbles (usually fairly evenly distributed around the circumference of the tire, but rather a big wad stuck on the inside of the wheel. I've had that before and it's like a 2 ounce wheel weight in the wrong place.

 

On your way out of the paddock try this:

 

Job done.The heat gun worked a treat[thanks Red9].Hardly any marbles but numerous BIG wads like I had run over several rubber pizzas!For interest the pieces of rubber from all four tires half filled a laundry bucket and weighed 19 ounces.My gut feeling is that my 'heat cycling' of the new tires didn't work and that most of the rubber was from my own tires.Any tips on proper heat cycling?

 

Ok, here is the trick you use to clean the melted rubber off of your tires.
Get a 5 gallon bucket of water and pour it in a circle slightly larger than the minimum turning radius of your car, Somewhere on a flat space in the paddock will do. Next drive the car slowly around in the circle of water in first gear. No need to get up any kind of speed. Something like 1500,2000 rpm will do. Circle around 10x clockwise and 10x counter clockwise and you are done.

The tires will be perfectly clean and have a very minimal graining to them.

 

Heat cycling has nothing to do with this and it's most likely not your rubber you picked up rather from other cars. Try a search over at RennList for heat cycle tips. When I start a new set of R-compound tires (no need to cycle streets or the 19" Michelin PSCup - it's not a real Cup), I put on my race pads and the new tires, go out and bed in the pads, and that process usually builds about 8 PSI meaning I got enough heat into the tires (they are hot to the touch). Leave them for at least 24 hours (I usually do this 48 hours before an event) and that's it.

 

Another thought. Your vibration may be unrelated to the marbles on your tires. Hot brakes on the track can cause the adhesive on wheel weights to soften and the weight can come off. Foil tape over the wheel weights is the solution to this problem. You might want to inspect the wheels to make sure a weight hasn't come loose. (Duct tape can also melt fyi)

 

As mentioned earlier, turning back and forth quickly on your cool down lap then a quick brush off when you get stopped is all that you need to do. I think anything else is overkill.

I wonder if waxing would help...