Polishing the rear softtop window of a convertible is not overly difficult, but might require a bit of trial to get the technique right. There are a few ways to handle this.
Why polishing a softtop window
The window in the softtop is most often made from a flexible but transparent type of vinyl. This material is softer then glass and is more susceptible to surface imperfections. The material can also suffer from UV damage and general discoloring. Oxidation has also been known to cause to issues with the clarity of the material. When this is not treated in time, the material can dry out and crack. This would create a leak for rain to get through. When putting down the roof, it might crack the vinyl. Obviously it can also be dangerous if your vision through the rear window is limited. Even for cosmetic reason it can be recommended to remove surface imperfections. Polishing is one of a few different ways to improve the look of the flexible rear softtop window.
How to polish the rear softtop window
There are a few different to approach this problem. The biggest issue is that the material is soft and flexible, making it difficult to put pressure on it. You can either use counterpressure or improve the working conditions.
Using another polisher
If you have somebody close by who knows how to handle this issue, you could polish both sides at the same time. As long as both polishers press against each other from both sides, with equal pressure, the vinyl will be polished on both sides with the use of pressure. This will take some practice as both parties would need to move at the same time, same direction and same speed with the same amount of pressure.
Using an object to counter
If another polisher isn’t an option, you might be able to use a flat surface to push back. Something like a flat piece of wood could work. However, you either need something to hold it in place or somebody to hold it in place for you. Be careful that the material you use won’t create surface imperfections on the vinyl. Wrapping it in a microfiber towel might help.
Removing the window
If possible, remove the roof will allow you to lay the window down on a flat surface (like a table), which will give you the counteraction to apply pressure with the polisher. This will make it much easier, but you would need to check the results carefully. When the roof is installed, the light going through the material will easily reveal imperfections. When laid down on the table, there is no light going through the material, making it less easy to spot these imperfections.
Polishing vinyl can be done with a regular polishing machine. Due to the effect heat has on vinyl it is recommended to keep the speed low, and make sure there is enough lubrication in the polish. A dual action polisher might make it easier to get a better finish. Because of the soft material you can get a good polishing result without the use of a cutting polishing pad. In some cases a finishing pad or a light polishing pad can be enough to get proper correction. The use of a soft polish can give better results because it will leave less of imperfection behind that need to be corrected by a lighter polish. Some manufacturers even make special vinyl polish, or plastic restorers (polish) for this task.