I have baseboard radiators in a new home (built in the 50s) and the fins are filled with lint. Vacuuming isn't pulling the stuff out. Is there a tool that can help with this?
What worked for me were Q-tips. I worked them around the surfaces of each fin and attempted to do the same with the pipe to which the fins are attached. The shaft of the Q-tips were long enough to reach beyond the top to bottom of the fins, and the tips were wide enough to touch both surfaces of adjacent fins without damaging them. The process was not fast but it was effective. The clean-up was with a vacuum cleaner. Subsequently I sprayed the surfaces with the Clorox cleaner/bleach product and again ran Q-tips over the fins. Based on the cleanliness of the tips after use, the process did not seem to add much value.
Like most projects, the undertaker isn't limited to just one tool. You have plenty of options to get those heat-sinks cleaned up.
If the heaters are really as old as the house foundation, which is likely considering this was a convenient heating method for older homes (pre-1960), first you need to remove the front cover. You might have already done that since you are able to navigate the vacuum nozzle into the heat sinks which I can't imagine doing with the panels still on. The vacuum should be used towards the end of the cleaning process, which is great to pickup loose material, but not so much to "pry" off this build up of dust in question.
Sure, you can use a small little air can, but I would save that step towards the end as well for some detailed cleaning. But to get the bulk of the debris out, I recommend a portable air compressor with a Pistol Grip Blow Gun. Similar to the air can, but with much more pressure and focused aim. If you don't have a portable air compressor, it's definitely a good investment and you will find many uses for them. Many compressors on the market today either come with a set of attachments or can be purchased separately.
For safety, of course make sure no one else is in the area, and that you wear a face mask to avoid breathing in any debris. Leaf Blowers aren't recommended indoors if they are gas powered. Even electrical leaf blowers aren't recommended as the air has a much wider landing area and they don't have as much power as one would expect.
Also, before attaching the front cover (and possibly side covers), if you notice that the fins are severely out of shape, you should use an appropriate tool to straighten them out if possible. Doing so is very easy as the fins are very flexible, hence the reason why they might be out of shape. But it may take some time straightening out fin by fin. The purpose of doing this is to provide a better heat transfer and better, well, radiation. But if they are just very slightly bent, you shouldn't worry about it too much. Technically it will reduce efficiency, i don't know by how much, but it's expected to never see these fins in perfect shape. They may even purchased brand new with slight bends here and there so It's not a major concern.
A vacuum and a paint brush do a nice job on this. Vacuum first without touching the fins, then use the brush to clean between the fins, catching the fallout with the vacuum. The bristles of the brush reach deep between the fins without bending them.
If necessary, after you get what you can dry, you can use plain water and see if you can get a little more. You could try using a solvent or cleaner but I'd hesitiate to use anything that leaves a residue or anything corrosive or flammable. That rules out just about everything but plain water.
Sounds like you need a small crevice tool adapter for your vacuum. You could also make your own pretty easily with some plastic and duct tape.
Here is another one:
I also have these from the 1950s in mine allow you to take off the front panels to clean them more easily. You might check if yours do as well.
Sometimes you just need some pressurized air to blow out the dust and lint stuck in the gaps/crevices.
Some canned air may work but personally I use a small leaf blower to clean out the fins, it works very well but sometimes gets everything else dusty, especially if the vacuum did a poor job removing most of the dust.