Waterbased Polyurethane over decking oil?
1. I have a couple of decks both at least partially exposed to weather. I gave them three coats of Minwax Decking Oil "Natural" which has a slight yellow tint for at least some UV protection. The tint is quite attractive in colour.
The timber for the decks is recycled mixed hardwood (60 yo old joists in one case and laminated fence palings in the other case). They were dressed and sanded. I filled as many cracks as viable with Sikaflex.
Well, the Minwax decking oil is next to useless for keeping out water and possum pee, so both sets of timbers have cracked and degraded within 6 months (where they get water sitting on them).
2. I have fairly recently had a handrail fitted to the back steps, and it was constructed from KD Blackbutt (not my choice). In this case I already knew that the MW oil was useless for water protection, so after 3 coats of that I applied several coats or water based Plyurethane to keep out as much water as possible. The finish was very nice and smooth and completely water resistant.....for a while.
However, I doubt the Blackbutt was properly dry (as is so often the case with big chain hardware timber), and it started cracking within a couple of weeks of being finished. The cracks somewhat defeated the purpose of the PU in keeping out water, and so the cracks got worse, particularly after the heavy rain of last weekend.
3. I had planned to take up both sets of deck boards and re-do them, applying PU (which is hell durable under foot). One of the deck boards had to be done before the rails went around the deck (BB rails as part of the hand rail). I sanded back and re-finished this board with 3 coats of oil, followed by 3 or 4 coats of PU. This particular board has been down for about 6-8 weeks now and shows no sign of deterioration whatsoever. It has been subjected to rain, sun and possum pee (which just wipes straight off without a trace). It has been in place for the same amount of time as the BB handrail which has started cracking apart.
4. As I was putting water based PU onto oil I had foreseen that it would not adhere very well without some sanding. The oil finish was lightly sanded with either 240 or 320 grit, and the PU went on very well without pooling or being patchy.
5. I am in the middle of doing the same treatment (oil + PU) to the 60 yo recycled cypress boards that are the palings ford the deck fence, going on to BB rails. Just to see what would happen I did a small area with PU over oil without sanding and it pooled as I expected. In this case using abrasive paper didn't work so well as it quickly got clogged making the disc useless after one or two boards (sander on lowest speed) so I used 00 steel wool instead, and have had good success in getting the PU to adhere. I left the oil finish to go off for about 8 days. It may be that I left the
single deck board to go off for up to two weeks - I can't remember.
So, the question is this: do you think that although the PU goes on successfully that it may still have long term adherence problems to the oil finish which won't show up for a while,
OR
is the good adherence an indication that it will be long term successful, and that the BB cracking is due to insufficient drying of the timber before being sold? It is my opinion that Spotted Gum would have been a much better choice than Blackbutt (I am going to do a test).
Sidenote: In the 6-8" of rain that we had last weekend that single deck board didn't even change colour a little bit, where the boards next to it (same species) went very dark with the water (the are oiled only).
