Wood sealant for greenhouse

[brianwelch]

We are about to embark on the assembly of an 8 x 10 greenhouse purchased thru Costco...Wood components/kneewalls with polycarbonate glazing panels. All wood has been given a coat of water based stain/sealant at the factory and has been routed out (after staining/sealing) for the insertion of the glazing panels. I would like to give a coat or 2 of sealant prior to assembly, focusing on the grooves as they will not be accessible in the future and will most likely be subject to moist conditions, but also on all mating surfaces/joints. According to the manual the lumber used is Cunninghamia lanceolata.
Thoughts/suggestions on a good sealer product is appreciated...
TIA

[branchacctg]

Cunninghamia lanceolata, a Chinese fir, is considered highly decay resistant, so may not need another coat or two of sealant

[brianwelch]

Thanks...
I only ask as the dadoes appear to have been cut after initial factory treatment, and the manual mentions re-sealing annually...appreciate the ionfomation tho'...

[slippy20]

Never heard of that species, but I looked it up...

C. lanceolata is a member of the family Cupressaceae (Cypress Family), making it more closely related to Cypress, Redwood, and Cedar than to the trees we consider Firs, which are in Pinaceae (the Pine Family).  So, it probably makes sense that this wood will perform more like the former than the latter.

Though, as my systematic botany professor always said in college, in botany, there is an exception to every rule.

I would use a penetrating oil base stain, like ones used for decks and fences. First coat thinned, then another coat, per the manufacturer.

[brianwelch]

Thanks Slippy...I was pleasantly surprised to find out a proper lumber was selected for the wood components (my usual luck would have me with poplar). My over-thinking mind was telliing me to stay away from oil based for 2 reasons...
1. Water based was already used, and 
2. The direct insertion of polycarbonate (aka plastic, to me) glazing panels into the routed lumber might not "play well" with an oil based product...

Appreciate the suggestion...