diff --git a/lib/gpu/README b/lib/gpu/README
index 900cffd35c..59510131d6 100644
--- a/lib/gpu/README
+++ b/lib/gpu/README
@@ -32,8 +32,13 @@ Makefile.lammps is created by the make command, by copying one of the
 Makefile.lammps.* files.  See the EXTRAMAKE setting at the top of the
 Makefile.* files.
 
-IMPORTANT: You must examine the final Makefile.lammps to insure it is
-correct for your system, else the LAMMPS build will likely fail.
+IMPORTANT: You should examine the final Makefile.lammps to insure it is
+correct for your system, else the LAMMPS build can fail.
+
+IMPORTANT: If you re-build the library, e.g. for a different precision
+(see below), you should do a "make clean" first, e.g. make -f
+Makefile.linux clean, to insure all previous derived files are removed
+before the new build is done.
 
 Makefile.lammps has settings for 3 variables: