A quick history:
I purchased this house in April, 2016. The west side of the spillover spa had some plaster discoloration above the step - it was mottled and a bit darker than the rest of the pool. I figured it was just stained in some way, but didn't worry about it. The plaster is potentially original to the pool, I don't know. It's definitely old and quite soft - soft enough that I can see wear from the plastic edge of the pool brush on the side of the spa just above the bench where that edge rubbed it when I was brushing the bench. The wear mark is about 1/16th to 1/8th inch deep. Since taking possession of the house, I've been quite careful with the chem levels using the TFP approach.
Late this spring, I had the pool and spa covered with a solar cover to attempt to get the pool near swim temperature. The pool itself looked fine - I was pulling back the pool solar cover to test and noticed no issues. CC stayed 0, no heightened chlorine demand, nothing out of order. I noticed one day that the color of the cover on the spa looked funky, and pulled it back to see a fine film of green algae on the side of the spa where the discoloration was previously. I SLAMmed, brushed the pool and spa daily with a nylon brush (see soft plaster - I'm not sure a steel brush is a good idea for regular brushing). I saw chlorine demand consistent with killing algae for two days, then passed an OCLT on the third day. All SLAM criteria were passed, so I went back to regular chemical levels. Algae returned in the spa within a few weeks, again, same place. Figured maybe I was dealing with mustard algae, so I did another SLAM (again, about a two day SLAM, then passed all criteria). I then slammed for mustard for 48 hours - all covers, pool toys well doused in chlorine, all towels, suits, etc. washed in hot water. Brushed vigorously. A week later - algae came back. Now I'm also seeing some reddish algae in the shady areas of the main pool near the steps/along waterline grout, etc. I've tested and retested CYA and have been keeping correct FC levels. I basically decided to brush and keep it under mild control, and finish out the summer before attempting to correct the issue.
So, normal mustard SLAMs aren't working. I'm now trying to craft a theory that will explain the issue and help me to take care of it. So here we are:
My theory is that the plaster is old and porous - it's pretty rough in places and very soft. I think the algae has a hold in those pores and my brushing isn't adequately removing the biofilm, hence the algae isn't killed and is able to come back despite a successful SLAM/Mustard SLAM Finish.
Experts - does this sound like a possible theory?
If so, I need to drain to reduce CH levels this winter anyways. My current plan is to drain, and while the water is low, gently sand the affected areas until I see unmottled plaster. Once all areas are addressed, refill the pool and perform a SLAM/Mustard SLAM. The cartridge is due for replacement too, so I'll handle that at the same time. Does this sound like a potential solution or am I missing a trick/not thinking correctly about the problem?
I purchased this house in April, 2016. The west side of the spillover spa had some plaster discoloration above the step - it was mottled and a bit darker than the rest of the pool. I figured it was just stained in some way, but didn't worry about it. The plaster is potentially original to the pool, I don't know. It's definitely old and quite soft - soft enough that I can see wear from the plastic edge of the pool brush on the side of the spa just above the bench where that edge rubbed it when I was brushing the bench. The wear mark is about 1/16th to 1/8th inch deep. Since taking possession of the house, I've been quite careful with the chem levels using the TFP approach.
Late this spring, I had the pool and spa covered with a solar cover to attempt to get the pool near swim temperature. The pool itself looked fine - I was pulling back the pool solar cover to test and noticed no issues. CC stayed 0, no heightened chlorine demand, nothing out of order. I noticed one day that the color of the cover on the spa looked funky, and pulled it back to see a fine film of green algae on the side of the spa where the discoloration was previously. I SLAMmed, brushed the pool and spa daily with a nylon brush (see soft plaster - I'm not sure a steel brush is a good idea for regular brushing). I saw chlorine demand consistent with killing algae for two days, then passed an OCLT on the third day. All SLAM criteria were passed, so I went back to regular chemical levels. Algae returned in the spa within a few weeks, again, same place. Figured maybe I was dealing with mustard algae, so I did another SLAM (again, about a two day SLAM, then passed all criteria). I then slammed for mustard for 48 hours - all covers, pool toys well doused in chlorine, all towels, suits, etc. washed in hot water. Brushed vigorously. A week later - algae came back. Now I'm also seeing some reddish algae in the shady areas of the main pool near the steps/along waterline grout, etc. I've tested and retested CYA and have been keeping correct FC levels. I basically decided to brush and keep it under mild control, and finish out the summer before attempting to correct the issue.
So, normal mustard SLAMs aren't working. I'm now trying to craft a theory that will explain the issue and help me to take care of it. So here we are:
My theory is that the plaster is old and porous - it's pretty rough in places and very soft. I think the algae has a hold in those pores and my brushing isn't adequately removing the biofilm, hence the algae isn't killed and is able to come back despite a successful SLAM/Mustard SLAM Finish.
Experts - does this sound like a possible theory?
If so, I need to drain to reduce CH levels this winter anyways. My current plan is to drain, and while the water is low, gently sand the affected areas until I see unmottled plaster. Once all areas are addressed, refill the pool and perform a SLAM/Mustard SLAM. The cartridge is due for replacement too, so I'll handle that at the same time. Does this sound like a potential solution or am I missing a trick/not thinking correctly about the problem?