You can use powder or fine frit to conceal the joints in fusing. This is most easily done before the first firing.
Fine frit can be made from the off cuts from your prepared glass, or you can buy powder of the correct colour. Where two colours meet use the darker or denser colour of frit or powder over the joint. Push the frit into place with a brush or stick to form a regular edge. You should heap the frit over the joint to allow for the reduction in volume when fused. This will not work as well on tack fusing as it does on full fused projects.