To set the context to this post, I got around to using this again at the weekend, refinishing my first ever substantial wood/DIY project - a pair of speakers which I built in my mid-20s around a Focal kit. They are built like a brick outhouse, but the oak veneer was glued on with contact adhesive and I think some (heavily, but unsuccessfully, discouraged) use of pledge type cleaner combined with the way the veneer was wrapped around curved edges had lead to some splitty-bubbles on the curves. Combined with the fact that these speakers are fantastically good fidelity and of big sentimental value, but also very unpopular in certain parts (often described as 'those coffins' etc) and a total renovation of our living rooms, they badly needed a refresh to avoid being cast out (me or them or both).
I have only used RPC once before, when I sprayed it to bare timber (with a stain on) without much prior thought or difficulty.
I agonised at length this time because by the time I decided to apply it (seeking the mirror gloss) I already had acrylate paint on them (Tikurilla Helmi). Rustins give dire warnings about using over other products. I have spent more time than I should have done poring over the various Rustins technical sheets and the advice on here, particularly in these threads:rustins-plastic-coating-straight-from-the-horses-mouth-t61190.htmlavoiding-brush-marks-in-rustins-plastic-coating-how-t94088.htmlrustin-s-plastic-coating-rpc-t89048.html
As there is a lot of conflicting information and some more dire warnings about the trickiness and issues that can arise I thought it might help to post my conclusions and results.
1. I had no problem going over the acrylate paint as a base. It was well-cured (well past its 7 day full cure period).
2. But, I sanded through to the primer on a corner on my final prep sanding - damn. I got some titanium dioxide pigment (120gm I think) and got most of it to suspend into circa 0.75l of RPC. If I was doing this again with more pre-planning I would look for a white universal colourant (ie the same, but in a liquid suspension) as the powder was quite hard to get suspended with enough opacity and wanted to stay as lumpy gritty sludge. But I got there with endless mixing and straining through paint filter cones. The result did not have immense opacity or covering power, but I had a solid new colour-base after about four coats (but added some more for flatting-off-ability).
3. Most importantly, timing wise:
A. The instructions say that the recoat time is 2 hours and that this window closes when a critical phase is reached after a temperature dependent period some 12-24 hours. After that, re-application will result in the hardened skin of the previous coat not being properly chemically fused with the new coat, but instead melting and sagging as a visible film under the surface of the new coat.
B. The 'horses mouth' thread suggests that a minimum of 3-4 hours should be left before reapplication and suggests that the application window does not close for much longer than the official instructions suggest.
C. There are many concerning warnings on here about trying to apply too many coats too fast (but not in the official literature).
D. As I was doing the application in the (not quite yet in use) living rooms, I needed to get many coats on this weekend to get this done and dusted as it was holding up moving in which is not domestically popular especially given the reason for the delay is these very damn coffins etc. On the other hand, this did mean I could control temperature by overriding the CH to a hideously sweltering 22.5c for the whole weekend - a big advantage in avoiding potential pitfalls. But, also qualifies the potential universality of this experience.
E. In light of my time constraints, I decided to go with the official instructions in terms of application times and ignore all the countervailing warnings. So, broadly speaking, 2 hour recoat intervals. That is apart from at night, with a 8-10 hour break, knocking the temp down to 18c (Rustins minimum temperature guidance) overnight to avoid the risk of accelerating the end of the window by too high a temperature.
F. So, I applied 3 coats of the white pigmented RPC on Friday evening (8pm, 10pm, 12). All fine, no sign of any issues (but the white being fairly opaque for all I knew at the time this might be lending false confidence)
G. Although the instructions and 'horses mouth' thread suggest that each coat should be rubbed down with 600g wet and dry and then wire-wool to key for the next coat, I figured that really this was inconsistent with the concept of these coats being a chemical bond, and so a waste of film-building. So I did not rub down between these coats. I did then flatten back at c10am on Saturday (400g on the RO sander, low speed and fairly gently) before three more white coats at 10.30, 12.30 and 2.30pm, followed by another flatting back, and a last white coat (which had some pearl power added) at 4.30pm.
H. 7pm and 9.0pm on Saturday, two coats of clear with pearl went on, then a cover coat at midnight.
I. Another clear cover coat applied at c10am Sunday, then another flatting back (had to wait until the pearl was well covered) before the start of clear coating properly at midday, 2pm, 5pm. Flatted back, more coats at 8.30pm, 9.30pm and 11.30pm.
J Flatted back at 8.00am yesterday and another coat then and at lunchtime, 4pm, 6pm and 9pm.
No issues experienced (yet, touch wood) with bubbling undercoats, sagging films or intercoat adhesion. So (subject to temperature) I am not sure that the doubt which has been cast on the Rustins official instructions is deserved. The 2 hour interval seems to be enough, and the concerns about applying too many coats in a day do not seem like they are necessarily warranted (again, so far, and subject to temperatures). I got up to 9 coats on in a day (and I think I make that 22 coats in 72 hours) without any such issues (yet, touch wood).
4. Application was by roller. I tried a few on the early coats - posh microfibre velour types, short pile Purdy, but the best by far for this stuff were humble No Nonsense foam rollers - they loaded with the right amount of finish and applied and smoothed the finish off superbly. You do have to be quick. So for me on each face it was: Zip, zip, zip three vertical rolls, zip zip zip zip etc crossways to get the whole surface covered, then slower and gentler passes all around the border edges then laying off (lightest pressure, barely resting the roller on the surface) with 5 slow gentle vertical passes and then crossways the same. Time is now running short. Inspect closely for minor runs and orange peely bits and can usually get away with a bit more finessing. if you start to see little stringy-snowflakey bits this is not the roller disintegrating (my first passing thought) but the roller pulling the forming film skin off - STOP!. They do seem to redissolve.
Not much else to add but I will update after polishing them up this weekend. Hope this helps someone in future who is thinking about using it.