Do you make a refractometer that can measure the concentration of rust preventative in water? The product we are testing for is known as RP-27.
The answer to your question is, generally yes. Refractometers are very good at measuring water soluble solutions, like rust preventatives. However, it depends on how soluble the rust preventative is and the detection limit or resolution that you require. Providing that the rust preventative is water soluble and providing that you are happy with a resolution between 0.1 to 0.2%, then a refractometer work for you.
Obviously we do not have a scale for every possible solution and it is a good bet that we do not have a direct reading scale for your rust preventative. In situations like this, we usually recommend a Brix refractometer like our 10430VP or our Palm Abbe PA201. Either will work. You will either need to obtain a Brix conversion factor from the solution supplier, or you will need to make your own.
To make your own conversion, you will need to carefully mix a couple known solutions that bracket your target concentration. Measure the known mixtures with the refractometer and chart the concentration vs. Brix reading on a piece of graph paper. Be sure to add a point for water which is 0.0 Brix. Draw a line through your points, or as close to all the points as possible. When measuring an unknown concentration in the future, just use the chart you created to convert Brix readings to the concentration of your fluid. Instruction for creating charts are available in the refractometer instruction manual.