Thompson's Water Seal
 

Last year I bought 2x6 cedar decking. I stained it (two coats) front and back and stacked it in my garage with spacer between each row for air flow. It stayed in garage for 9 months. Two weeks ago I played the deck. Then I used thompsons clear water seal to finish it off. After the speaker started to dry I noticed a oil looking substance laying on the boards in spots. It wipes up clear and has no odor. Do you have any idea what's happening.??

Thompson's Water Seal
 

On Sunday, July 26, 2020 at 9:45:59 PM UTC-4, wrote:
Last year I bought 2x6 cedar decking. I stained it (two coats) front and
back and stacked it in my garage with spacer between each row for air flow.
It stayed in garage for 9 months. Two weeks ago I played the deck. Then I
used thompsons clear water seal to finish it off. After the speaker started
to dry I noticed a oil looking substance laying on the boards in spots. It
wipes up clear and has no odor. Do you have any idea what's happening.??

You "played the deck" and then the "speaker started to dry"?

Sounds like you installed the sound system wrong.