How do I know if the waterproofing has worn off on my convertible?
Quick answer: Don't judge it by beading alone -- check absorption. After rain, if the hood feels sopping and is soaking up water, the waterproofing has worn off and it needs re-coating. With a healthy coating, even after a heavy downpour the fabric should only be damp. Beading often fades earlier as dust breaks surface tension, so that on its own isn't a failure.
The test isn't whether water still beads -- it's whether the fabric is drinking the rain in or shrugging it off.
How a fresh coating behaves
The whole point of a weather-proofer is that it's a hydrophobic coating and repels water. On a freshly coated soft-top, water beads up into little balls and rolls off.
Why beading fades before the coating does
Over time, dust and dirt accumulate on the fabric and break the surface tension that causes the water to bead. When that happens, rain will slip through the outer fibres and drip down onto the neoprene sheet underneath -- the layer that actually keeps the water out of the car. So loss of beading on its own doesn't mean the coating has failed.
The absorption test
If the water is no longer beading, feel the convertible roof after rain. If it's only damp to the touch, the hydrophobic coating is still working and stopping the fibres holding water. If it's sopping wet and clearly soaking rain up, the coating has gone and the fabric needs re-proofing.
Don't leave it going into winter
Re-coat before winter if you can. A saturated hood that freezes solid goes hard and brittle, and is much more prone to rips and tears. With the coating intact, even a heavy downpour should leave the fabric no more than damp. For more on what winter demands of a soft-top, see our notes on driving a convertible in winter.