GR has a wonderful solution for posting spoilers. At the top right of the comment box you'll see "(some html is ok)" and you'll find the instructions. I'll post the format here but will have to add spaces so you can see it.
< spoiler > your message < / spoiler >
Eliminate the spaces and it will look like this if you've done this correctly:
< spoiler > your message < / spoiler >
Eliminate the spaces and it will look like this if you've done this correctly:
(view spoiler)[your message (hide spoiler)]
It's the same format you'd use to make some text bold but instead of the letter "b" you type the word "spoiler."