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If kerosene is so much cheaper, don't you think that paint manufacturer would just use it and pocket the differece?

The naptha is there for a reason - it's a more powerful solvent.

I can tell you from experience cleaning bottom paint off of paint brushes that mineral spirits does NOT fully clean the brush. I need to use the naptha-containing brushing thinner to dissolve all the paint off the brush.

You can experiment all you want, but you'll just be reinventing the wheel. The paint manufacturer spent thousands of hours of R&D time coming up with the right composition.

If you really want to save money by making your own thinner, at the very least you should duplicate the composition that the mfg reports on the MSDS. Then you'll have to buy some naptha and blend it into the kerosene at the 25% level.
 
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...I imagine naptha will be difficult to locate in the retail market...
Naptha is available at Home Depot.
...The msds for Interlux 333 provides a range of percentages for the composition of kerosene, 75-100%, which I took to mean that sometimes they use 25% naptha and sometimes not. But that could be the wrong assumption...
They "company line" is that reporting a wide range allows for variability in the product. The real reason is they don't want to give you the exact composition to prevent you from using the MSDS to formulate your own product instead of buying theirs. ;)
 
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It bugs me that the paint manufacturers repackage a mixture of common solvents and then increase the price 400%. $52/gal for Interlux 333 vs. $11/gal for kerosene is a substantial difference. If I could buy only the amount that I need, then you are correct that it is a relatively small investment; but that I must buy at least 1 qt. (333 sells for $32/qt.!), which will sit in the can for several years seems like a waste of money. I appreciate your thoughts on the subject - thanks! B.
Short answer: You own a boat. $32 is one of the least expensive things you'll buy for your boat, and a lot less than the paint that it is going into. And a quart is about the smallest quantity that it is feasible to package. When you add in the usefulness of this material for other things such as dewaxing your hull and cleaning your brushes, you'll find it to be money well spent.

Long answer: You are paying for a lot more than the cost of raw materials in that can. You're paying for the R&D that goes into determining the optimum blend of solvents to work in a broad variety of paint compositions and environmental conditions. You're paying for the tech support guy that you called at Interlux. You're paying for the quality control that ensures that the can you buy today is the same as the can you bought two years ago. You're paying for all the regulatory requirements for the manufacturer to ensure that the product is safe in use, during shipment, and when disposed of. You're paying for the implied warranty that using genuine Interlux thinner will insure that you don't ruin the much more expensive Interlux paint that you're adding it into.

Nobody is stopping you from using the kerosene. Go ahead and take your chances. But if you want assurance that the product will work right, you might consider paying the $32 for that assurance.

Disclaimer: I've spent my entire career doing R&D for paint and ink manufacturers.
 
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