  We propose a two-step procedure to study the order of phase transitions at
finite temperature in electroweak theory and in simplified models thereof. In a
first step a coarse grained free energy is computed by perturbative methods. It
is obtained in the form of a 3-dimensional perfect lattice action by a block
spin transformation. It has finite temperature dependent coefficients. In this
way the UV-problem and the infrared problem is separated in a clean way. In the
second step the effective 3-dimensional lattice theory is treated in a
nonperturbative way, either by the Feynman-Bogoliubov method (solution of a gap
equation), by real space renormalization group methods, or by computer
simulations. In this paper we outline the principles for $\varphi ^4$-theory
and scalar electrodynamics. The Ba{\l}aban-Jaffe block spin transformation for
the gauge field is used. It is known how to extend this transformation to the
nonabelian case, but this will not be discussed here.
