Do homemade convertible top cleaners work?
Quick answer: Homemade cleaners may shift light dirt, but we don't recommend them. Vinegar, washing-up liquid, bicarb, etc. can strip waterproofing, leave residues that attract dirt, or mark the fabric. Use a dedicated soft-top cleaner, rinse thoroughly, repeat until truly clean, then re-proof. If you must DIY, stick to mild pH-neutral shampoo, very dilute, and spot-test first.
A kitchen-cupboard mix will never beat a purpose-made convertible roof cleaner. The chemistry in dedicated hood products has decades of detergent science behind it, and it is aimed squarely at the organic grime that grows on a cabriolet fabric. Specific problems like tree sap baked into the weave need targeted chemistry; most kitchen-cupboard fixes won't touch aged resin.
What people reach for first
Before getting a dedicated cleaner, most people try something they already have. Washing-up liquid is the most common. Vinegar comes up a lot for algae and green growth. Some try bicarbonate of soda on stains, or carpet shampoo on the theory that it is designed for fabric. We have seen all of these in the workshop, and each one creates a slightly different problem.
Washing-up liquid strips the waterproofing
Washing-up liquid is a degreaser. That is exactly what makes it effective in a kitchen, and exactly what makes it damaging on a soft-top. Modern convertible roofs are treated with a DWR (Durable Water Repellency) coating that makes water bead and run off the fabric. A strong surfactant cuts through that coating in one application. The fabric may look cleaner, but it loses its water-repellency immediately -- so the next rainstorm soaks straight in instead of running off.
There is also a residue problem. Washing-up liquid is designed to be rinsed off hard, non-porous surfaces. The open weave of a soft-top holds onto surfactant residue even after what seems like a thorough rinse. That residue attracts dirt, which is why roofs cleaned with washing-up liquid often look dirtier than before within a few weeks.
Vinegar and algae removal
Vinegar does kill algae and lichen -- that is genuinely true, and it is why it comes up as a recommendation online. The problem is that it works aggressively and without discrimination. Acetic acid can permanently bleach or discolour canvas and vinyl, particularly on coloured roofs. On a black roof the damage may not be obvious straight away, but the fabric becomes more porous after repeated acid exposure, which makes it both harder to clean and more prone to soaking through.
For algae and green growth, there are better approaches that use biocidal treatments pH-balanced for soft-top fabric. They remove the algae cleanly without the side effects. More on why vinegar is a poor choice for lichen removal is covered separately.
What happens to the waterproofing
The deeper issue with any non-specialist cleaner is what it does to the fabric's water repellency. A roof that sheds water effectively also resists staining, mould growth and UV damage. Strip the DWR treatment with harsh chemistry and you lose that protection -- often permanently unless you re-treat immediately afterwards.
If you clean with a household product and do not re-proof the roof straight after, you are leaving it in a worse state than when you started: cleaner on the surface, but structurally more vulnerable.
If you have nothing else to hand
If you need to shift surface grime and have no dedicated cleaner available, the least damaging option is a pH-neutral car shampoo, diluted further than the label suggests, applied with a soft brush and rinsed off thoroughly with cold water. Spot-test on an inconspicuous area first. This is the minimum viable option, not a recommendation -- but it is better than reaching for washing-up liquid.
Do not use anything strongly alkaline or acidic. Do not use carpet shampoo, upholstery cleaner, or anything containing bleach. The full guide to cleaning a soft-top roof covers what dedicated products do differently and why the process matters as much as the chemistry.