Under ultra high vacuum (UHV) conditions tin (Sn) forms a monoatomic wetting layer on aluminium (Al) surfaces if Sn-islands formed by a preceding deposition process are present. Previous experimental observations and Kinetic Monte Carlo (KMC) simulations suggest that wetting layer formation is governed by thermally activated surface diffusion and adsorption processes.This paper presents a systematic study of the wetting of the inner and outer interfaces of Al by Sn in sandwich systems consisting of a 400nm Al-base layer, a 10nm Sn interlayer and a 400nm thick Al capping layer. The morphology and chemical composition of the sandwich systems is investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The wetting process is studied by scanning auger electron spectroscopy (AES) under UHV conditions. Depositing the Al-capping layer at different deposition rates allows for the assessment of the influence of the grain boundary density on the velocity of Sn-transport through the Al-capping layer. Studying the permeation speed of Sn through the capping layer at different temperatures shows that Sn penetrates the capping layer much more rapidly at elevated temperatures thus corroborating the involvement of thermally activated mechanisms in the transport process.