One of the investment themes that could come to the fore following the EU referendum is how to Brexit-proof your portfolio. In other words, protect it against the potentially damaging effects of the UK existing outside the EU.

Clearly, there’s no guarantee that the UK will perform any worse (or better) outside of the UK than it has done while being part of it. But as far as investment themes go, the idea of making a portfolio Brexit-proof may prove popular among nervous investors.

Long-term play

One way to reduce the potential effects of Brexit is to invest in companies that operate internationally. One such stock is PZ Cussons (LSE: PZC). It has major exposure to Nigeria which remains its key market, as well as Asia and other parts of the world. Therefore, PZ Cussons may be hit far less hard by the effects of Brexit than is the case for most of its mid-cap peers.

However, PZ Cussons faces its own problems in Nigeria. The economy is enduring a highly challenging period and PZ Cussons’ sales and profitability figures have disappointed of late. As a result, its share price has fallen by 10% in the last year and there could be further falls to come in the short run if Nigeria’s economic problems continue.

For long-term investors though, PZ Cussons remains a sound buy since it trades on a price-to-earnings growth (PEG) ratio of 1.9 and undoubtedly has huge growth potential from a rapidly expanding emerging world.

Stability option?

Also offering international exposure is National Grid (LSE: NG), with the utility company having operations in North America. Clearly it remains UK-focused but due to its lack of reliance on the prospects for the UK economy, its performance looks set to be stable and resilient in future.

One cloud on the horizon for National Grid is rising interest rates. Inflation could spike due to a weak pound, which makes imports more expensive. And due to National Grid’s debt pile being high, its profitability could be squeezed by high debt servicing costs. Countering this, though, is National Grid’s vast defensive appeal so that if inflation and interest rates rise, it could still be viewed as a worthy buy among more risk-averse investors.

Shaking off Brexit

Meanwhile, Aviva (LSE: AV) has stated since the referendum that its outlook isn’t set to be significantly affected by the UK leaving the EU. This is excellent news for the company’s investors and yet Aviva’s shares came under severe pressure in the days following the vote. This could present a buying opportunity since Aviva’s integration with Friends Life seems to be progressing well and the life insurer now trades on a price-to-earnings (P/E) ratio of just 9.

Looking ahead, it would be unsurprising for Aviva’s shares to beat the wider index given their low valuation. However, their outperformance could be substantial because Aviva is forecast to record a rise in earnings of 8% next year. And with Brexit unlikely to significantly impact on its business in future, the chances of Aviva meeting its guidance remains high. This positive outlook and a wide margin of safety make Aviva an appealing post-Brexit buy.

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Peter Stephens owns shares of Aviva and National Grid. The Motley Fool UK owns shares of PZ Cussons. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.