I would begin by phoning one or more vinyl siding manufacturers and
presenting your question to them. They are the experts.
I would also try to determine what the real problem is - the fact that some
of the siding is shiny or the fact that the rest of the siding is dull? You
could consider restoring the sheen to all of your siding using a powerwasher
or using one of the commercially available products for washing vinyl siding.
Once again, the manufacturers should be able to advise you.
If you decide that you need to degloss parts of the siding and none of the
manufacturers have any good advice, then here's what I would suggest:
1) Get some scraps of vinyl siding. You can get plenty of small pieces if
you can locate a vinyl siding job in your area and politely ask for some
waste pieces from the scrap pile. Please respect the contractor, his workers
and his job site - don't disrupt a job site without asking.
2) Experiment on those pieces with a variety of common chemicals around
your house - isopropyl alcohol, denatured ethanol, methanol, various painter
thinners, fuel oil/kerosene, charcoal lighter fluid, WD-40, soap & water,
house-hold cleaners, gasoline, etc. I'd prefer to wipe on an extremely light
coat and I'd try to remove the solvent as soon as possible. From my past
experience with any sort of chemical deglosser on any surface - the deglossing
occurs almost instantly and any additional time can contribute to degrading
the surface. Observe results a day or two later.
3) Note that you may need to use a brush if you are attempting to degloss
the depressions in the siding as well as the high spots (I assume that your
siding has a simulated wood texture.) Obviously, most abrasive methods
such as sandpaper are going to leave the depression areas still glossy.
4) Try the safest successful solvent on one spot on your siding and observe
the results a day or two later. If your happy, then take care of the other
Coming up with you own method to degloss the siding could be tedious.
Some chemicals won't degloss. Other chemicals are capable of removing
the plasticizers from your vinyl siding. The gradual loss of plasticizers in
vinyl siding and in asphalt roofing shingles is the major reason that these
items eventually turn brittle. Obviously you don't want to accelerate this
process on your vinyl siding. Also, many chemical are capable of etching
the vinyl - you don't want to eat through the siding or reduce its thickness.
I emphasize again - I'd attempt to research this a lot before attempting to
come up with my own method of fixing the problem. There is little need
to reinvent the wheel.