Spider control:
We have something like 60 different spider varieties around here, they don't all try to live in my rain bucket fortunately. According to my exterminator, there are no products that will work to repel the little beasties. However he gave me an old wive's tale about repelling spiders, he said to vacuum up the spiders inside the bucket into a catch chamber (had to buy this, it goes into the hose and collects anything sucked up). Take the live or newly killed spiders and crush them to make a paste, add a few drops of water to make it a little soupy. Use this spider soup to paint around the openings inside the rain bucket. The spiders coming in will smell the stench of dead spiders and flee according to the old wives that came up with this one. I haven't tried it yet.
I have tried a tick and flea spray that my vet sold me to control those buggers on my pets, so far it has worked on the rain bucket. The product is made by Adams and I've been using it to kill bugs for years, it works on anything it hits, including spiders. I respray the inside of the bucket every two months. So far this year only a couple of tiny spiders have been found.
A suggestion on the spider silk clogging the anemometer. I think this is all in the location of the sensors. In this area we have spiders whose young will climb to a high point and use spider silk as a parachute to help them disperse after they hatch, a lot of that silk gets stuck to the launching points like an anemometer. If you mount the sensors on a tall enough poll that's high enough they won't make the climb. You can also place pigeon spikes around the pipe a couple of feet below the sensors. This way the spiders will launch or nest in the spikes instead of the sensor. If the sensors are easily accessed, you can also use a garden flame thrower, give the sensor a very quick blast of flame and the silk is gone.
Food for thought!
Doc