To the Composites Forum,

I'm very appreciative you exist as I might now be able to avoid some mistakes in the near future with some help.

I am coating three industrial size concrete sinks with XCR Clear Epoxy Coating Resin (photos below) that are 98% free of pin holes and sanded up to 120 grit. The sinks are abut 270 cm L  x 70 cm W x 25 cm D and we used a 50% sand aggregate to 50% cement mix. It is very dense material. Several of the surfaces are vertical and other surfaces uneven including some rounded channels. We are hoping to do the epoxy coating in April and the sinks are now outdoors, but under a permanent rain free shelter (as pictured). As outdoor April temperatures in West Sussex will most likely not be able to be maintained at a 15 C minimum for a 24 hour period (and humidity quite variable), we plan to create an insulated structure around each sink, use a heater below the sink for 24 hours before applying the epoxy and paint the epoxy with three to four coats at around 15 C to 25 C. We plan to leave the heaters on during the epoxy coatings and the initial cure for 24 to 48 hours later (but perhaps a longer heating time may be needed).

So, does this sound like a good plan to deal with the temperature and humidity issues? How long do the sinks need to stay at least 15 degrees during the initial curing? I understand full cure takes 4 weeks, but I'm hoping this longer cure time does not need a constant temperature of 15 C.

This will be my second epoxy application, so I am quite new to this. The first attempt I made was on a test piece of white concrete (see first pic below). The concrete was not vibrated and has a significant amount of pin holes (unlike the sinks). I kept the test piece vertical as shown and painted two coats of XCR with a paint brush (starting from the bottom and moving upward). Also, I sanded the top half (not the bottom) before applying the second coat. Finally I applied a third coat to just the bottom half which meant that the bottom half was left unsanded until the end. The coats were fairly thin, painted on at around 22 C and cured in a room ranging from 25 C to 30 C. 

I was satisfied with the gloss result (even after initially sanding the first coat on the top half) and was quite surprised that almost all of the vertical pin holes were filled in by the epoxy. BUT, either the paint brush seemed to have left streaks or the many tiny craters created by the small and large pin holes created this unevenness. One very small area without holes seems to have a flatter finish but not completely flat. I am guessing the unevenness is mostly caused y the crater holes rather than the brush strokes. Any insights?

I used about 250 grams of XCR Epoxy on the small slab for 2.5 coats (The slab is aprox. 30 cm x 60 cm). And I estimate the coating is about 1.5 mm to 2 mm thick, but that is just a guess. So we purchased 10 kg of XCR for all three sinks, I do not know if this is enough for three coats (at least to total 2 mm thickness). I will see in April after we do the first sink.

Any advice on application would be most helpful especially in applying coats vertically: whether to use brushes, rollers or squeegee or all three. I am interested to know how to pour it on or where to paint first, using thin layers or the traditional pour over method. If painting a thin layer, should I start from the drain hole and paint upwards or paint from the sides down to the drain hole? My initial sense (if using a paint brush) is to paint from the lowest surface up as I did on the test piece.

Many professionals working with epoxy coating sail boats swear by the squeegee method (after an initial thin coating to seal the outer surface you pour large amounts of epoxy on this first thin coat then squeegee the epoxy all over hoping it settles evenly -- but this is with a single flat surface). Others prefer paint rollers but caution about the correct roller that does not leave debris in the epoxy by falling apart in anyway. Since I have to coat vertical and horizontal surfaces together (as well as the splashback), I think this needs to be handled in a nuanced way, but maybe I am wrong?

Some people say they always sand through the coats, others avoid this. If I sand, what grit sand paper do I use (we do not own a wet sander, we only have an orbital disc sander with three speeds and a multitool). I also own some buffing pads that attach to a cordless drill.

If you have a tried and true method for such a complicated surface, please let me know. If there is a way to avoid sanding I would prefer it, especially to avoid paint strokes showing up or unevenness in the coating, but maybe with all of the vertical surfaces and the groves and, rounded edges, doing this without some sanding is impossible?

One initial issue is that the sinks are in place now and a bit over a meter above ground. So when we coat them, we cannot coat the outermost facing vertical surface until the end as we will need to be leaning against this vertical area to reach the other parts of the sink. Is that going to be an issue with waiting to coat this area at the very end? Any suggestions?

Lastly, the splashbacks are slate and thus a bit dusty/sandy compared to the polished concrete. We plan to epoxy the splashbacks as well, do you foresee any issues with doing this? Shall I coat them separately or altogether (first or last)?

Well those are the questions I have for now (quite a lot). My first epoxy test came out quite well despite the 1000's of pinholes and uneven surface, but the temperature and humidity were under control so it was a bit of a false success in certain ways as the sinks are in a different place outdoors and a different animal altogether with all of the twists and turns on their surfaces (not to mention that they are much bigger than this little piece of white concrete).

If you read through all of this, you are already a saint as it contains way too much information.

Thanks for the consideration,
Cunda (pronounced Chewnduh) 




[First test on white concrete (above)]



[All three concrete sinks in place -- with slate backsplash]



[Detail of Sink Bed]



[Detail of drainboard and splashback]



[Detail of grouting and slate splashback]



[Detail of exposed aggregate sanded down and a few rare pin holes on the vertical surface]