You can remove the strut assemblies from your 1999 Mercury Sable sedan without a spring compression tool, but to disassemble the strut assembly itself, you need the tool. However, strut-and-coil spring assemblies are available as replacement parts for your Sable. If you replace the strut assembly as a unit with the spring already installed, you don't need the compression tool.
When reassembling the strut assembly, use a new locknut at the top of the strut and tighten it to 39 to 53 foot-pounds. Use a crow's foot on the nut and use a box-end wrench to hold the strut rod from turning. To install the assembly, tighten the three 13 mm nuts on the front mounting brackets to 22 to 29 foot-pounds, and the steering knuckle bolt to 73 to 97 foot-pounds. Tighten the nuts for the rear mounting brackets to 19 to 25 foot-pounds, and the wheel spindle bolt to 50 to 67 foot-pounds.
