Thompson's Water treatment For Decks, Or... ?

Hello:
Have a deck that is made (thank goodness) from pressure treated wood. Deck is about 20 years old.
Will be wanting to apply some product like Thompsons Water Seal to it.
Questions:
a. Is Thompson's the brand to use, or are there "better" products available ?
b. How is this stuff applied ? Just swabbed on with a big, cheap, paintbrush, or... ?
How about spraying it on ? If so, would you still need to smooth the liquid with a brush ?
c. The underside of the Deck is high enough to walk under.
Should I also apply this stuff to the underside ? Brush or spray ?
d. How does one get the sides of the planks, seeing that there is about an 1/8 inch gap, only ?
Important to do the sides too ?
e. Does pressure treasted wood absorb any of this product, or is it pretty much of a surface treatment ?
Just out of curiosity, how about regular wood: surface only ?
Thanks, Bob
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Thompsons is not a good product , Consumer Reports has an ongoing test of many different types and dropped Thomsons because it does so poorly, actualy the main ingrediant is wax. If it is treated why seal it it does not need it. If you want to color it that is different, but if you like the color now just clean it.
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You get what you pay for, and Thompsons is cheap. That said, I have a dock on a lake. Your deck probably is less exposed and gets less traffic. I have treated it 3 times in the 6 years I have owned it, using the cheapest stuff I could find. Once Thompsons, once something from KMart, and once something from BJ's. The first time it sucked up the solution like crazy; presumably the previous owner hadn't treated it. After that it really didn't need it, as water still beaded up showing the coating was still effective, but I thought 2 year is long enough to go. I had time, and the stuff is cheap. So, while Thompsons is not the best, it lasts more than 2 years on my dock.

cheap enough not to worry about. A brush is good for corners. I have sprayed it, but the roller is easier and faster.

time, especially if it will get wet; but it just isn't worth the effort.

enough slop from the roller to get the sides. End grain is the most important though. Try to get all the end grain several times.

have treated, it is pretty much a surface treatment.

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20 minutes then power washed it off, that did a great job, better than just power washing alone. However, I had heard that Thompson's water seal was not good so opted for Olympia (or is it Olympic?(cans are gone) clear deck sealant which contains linseed oil and that did a great job. I used a sprayer for that. Did the immediate area around the house siding with a brush so as not to get it on the vinyl siding. Any that does happen to get on the siding can be cleaned up with mineral spirits. My decks kind of just sucked it up as it had been weathered for so long. All three decks look new. It was gray and dull and against the new siding looked like crap. I thought about using one of the tinted deck treatments then saw the neighbor struggling to scrap and sand it off to redo his.
Stone
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