Stripper Failure in a School Cafeteria
At 22:00, a two-person crew begins stripping the 1,150 m2 vinyl composition tile (VCT) cafeteria floor of an elementary school. The floor carries about 14 coats of finish applied over three years, with yellowing and black heel marks. The manufacturer's label lists heavy buildup removal at 1 part concentrate to 4 parts water; the crew mixes 1:32 with water at 18 C. They apply the solution with a flat mop, allow a 3-minute dwell, scrub with a 175-rpm swing machine and a black stripping pad, then wet-vacuum the slurry. In the last third of the room the slurry dries before pickup; the crew adds water from a bucket and continues. After one rinse, the floor dries with patches of smeared finish and visible finish in the grout lines. The building opens at 06:30, and the crew has 4 hours of labor left. Ambient conditions are 20 C and 55% RH.
1. Identify the most likely reasons the strip failed, and give a recovery plan that fits the remaining labor. Prioritize the areas to treat first.
Read the worked answer
- The mix is the first failure: 1:32 is eight times weaker than the specified 1:4 heavy-duty dilution (1 part concentrate plus 4 parts water). A 3-minute dwell is also short for 14 coats, and 18 C water slows the chemical action.
- The dried slurry in the last third was re-wetted with plain water. That dilutes the active stripper and can redeposit softened finish, which explains the smears.
- One rinse is not enough for that much residue.
Recovery: do not apply finish over the failed areas. Section the room into 50-75 m2 blocks. Mix fresh 1:4 with the warmest water allowed by the label, apply generously with a mop, keep the surface wet, dwell 5-10 minutes, test with a putty knife, agitate with the swing machine, wet-vacuum, then rinse twice with clean water and wet-vacuum again. Treat the entrance, serving line, and main aisles first because they are visible and high traffic. With four labor-hours left, two people cannot correctly strip 1,150 m2; the realistic choice is to barricade the unfinished area, complete a safe partial section, and schedule the rest rather than seal in a failed strip. Edges and grout lines need hand detailing with a narrow brush.
2. How would you verify the floor is ready for new finish? Include pH and dryness checks and the consequence of skipping them.
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After the final rinse and vacuuming, let the floor dry, then wipe a damp area with pH paper. The reading should be within the finish manufacturer's range, commonly neutral to mildly alkaline, about 7-10. If it is higher, rinse again; if an acid neutralizer is allowed, use it only per label and rinse thoroughly. High alkalinity can cause poor adhesion, haziness, powdering, and loss of gloss. Check dryness by taping a 1 m2 clear plastic sheet to the floor for 30-60 minutes; condensation means moisture is still coming off. Also confirm the floor is free of stripper film by rubbing a white cloth over a test area. Recoat only when the surface is dry and clean. Skipping pH or dryness checks can trap residue under finish and force another strip.
3. Calculate the concentrate needed if solution coverage is 25 m2/L per wet pass and two passes are required over 1,150 m2. Show the math.
Read the worked answer
Total solution = area / coverage per pass x passes = 1,150 / 25 x 2 = 92 L. At 1:4, total parts = 1 plus 4 = 5, so concentrate = 92 / 5 = 18.4 L. Mix 18.4 L concentrate with 73.6 L water to make 92 L. In practice, mix in smaller batches so the solution stays fresh and the crew does not over-apply a batch that has been sitting.