I've been living in Southern Thailand for the past two years, which means I've also been through two rainy seasons. In July and August we definitely get a lot of rain, but it does not normally last all day. There is still plenty of time to travel around and experience Thailand; plus in my opinion, it's quite a nice break from the heat. Also if you enjoy kite surfing or things like that, it's the perfect time due to all the extra wind. Hope that helps!
If you are interested in doing some volunteer work during your time in Thailand, please come check us out at
http://www.tmvolunteers.org/
Have a great trip.